What does it mean if a MacBook Pro has 16 cycles on the battery?

macOS battery charge cycle count indicates how many times the battery has basically gone from 100% to 0% to 100% which would indicate a single charge cycle. If a laptop is used from 100% to 50% to 100%, then that is considered half a charge cycle. If the laptop goes from 100% to 50% to 100% a second time, you now have added yet another half cycle.

Battery charging cycles have no real meaning unless you know how the laptop was used. A laptop which has been plugged into the charger most of the time will have a very low cycle count, while a laptop which is used on battery power every day will have a higher cycle count. The low cycle count where the laptop has been using the charger most days does not truly show the health of the battery. Age of the battery is more important when trying to gauge its health since the metals & chemicals within the battery degrade over time so a 5 year old battery with low cycle count may be nearing the end of its life.

Other factors affecting battery health which don't get recorded are whether the battery has been exposed to excessive heat conditions which degrades a battery, plus whether the battery has experienced any physical impacts or severe jarring since than can affect the very thin & fragile internal connections inside the battery which can cause them to degrade faster than normal. If the battery has been replaced with a third party battery, then who knows how healthy the battery is since the quality of third party Lithium batteries is extremely poor even when purchased from a reputable vendor. If the charge cycle is nearing 1,000 cycles, then it does indicate the battery is well used and likely needs to be replaced. That is the only value charge cycles have. Low charge cycles are irrelevant for understanding battery health.

The best you can do to judge battery health is using a third party app like Coconut Battery which will tell you the battery's current Full Charge Capacity (FCC) in relation to its original Design Capacity. If the FCC is less than 80% of its original design capacity, then many people will find the battery may not have enough capacity to run long enough for their needs, but other people may be Ok until the FCC get nearer to 75% of the design capacity.

Usually when the FCC value drops below 80% of the original Design Capacity, macOS will change the battery condition from "Normal" to "Service Recommended". The "Service Recommended" battery condition may also indicate macOS has detected a hardware fault with the battery.

Even if the Battery Condition is "Normal" and even if the FCC is greater than 80%, it still does not mean the battery is healthy. A battery can have a bad internal cell which can cause issues such as a sudden drop in usable capacity, but this is not normally monitored by any apps or the OS. Most people will see this occur on older batteries when the battery is nearing 20% charge remaining (usually 10% or less) where you think you may have lots of time before needing to connect a charger, but then suddenly the laptop powers off completely. When you connect the charger & boot the laptop you discover the battery charge level is now at 1%.

FYI, be very careful buying used Macs from people you don't know. It is very risky. If the Mac has a firmware password lock on it (Intel Macs only), then there is no way you can remove it ever....only the original owner can remove (or have Apple remove) it. Another concern with a used Mac is whether a previous owner (not necessarily the person selling the laptop currently) still has control of the laptop. These forums are full of posts where a person buys a used Mac only to discover a message popping up on their screen telling them about the laptop being managed. Many times these are Macs which were used by a school or business and were being managed by an MDM which can force the laptop to call home to a previous owner, where the previous owner can install software, remove software, impose restrictions, and even lock the Mac. There is nothing you can do to remove either one of these things. So be careful.

Also, here is an Apple article for what a person should do prior to selling a Mac:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If you do purchase a used Mac, then I highly recommend you perform a clean install of macOS even if the previous owner has already done so. For one, at least you will know it was done correctly. Secondly, it can reveal whether a firmware password lock is enabled, and many times you will see a management notice screen during initial setup (not always....in that case it may take hours, days, or even weeks before you may see a system notification -- the tiny ones on the side of the screen--- indicating something about being managed or a management update). A clean install is when you erase the drive before re-installing macOS. How you do this varies slightly between older Intel Macs and the newer Apple Silicon Macs (M1 & M2). Plus 2018+ Macs have a firmware "Restore" option as well which can be used if you have access to another Mac running macOS 12.4+. Having a clean install of macOS mean you know all of the previous owner's data & apps are gone and you have a factory fresh OS which should work perfectly if the hardware is healthy.

How to reinstall macOS - Apple Support

Revive or restore an Intel-based Mac using Apple Configurator - Apple Support

Revive or restore a Mac with Apple silicon using Apple Configurator - Apple Support