What does 128 SSD mean? Is it upgrade-able? What year model would a MacBook be more or less if it's still uses SSD?

SSD's are the current type of hard drives in computers. Apple Computers use these types of drives now, with some having the option to choose a regular / older type mechanical hard drive instead. Or a combination of both, known as a Fusion drive.

The number is the size. 128 refers to 128GB. Depending on what model of Mac we are talking about you can choose to upgrade to different sizes during purchase.

If this is a used Mac, then the upgrade possibilities will depend on the exact model year of the Mac.

Beyond that, while it's technically possible to upgrade SSDs in Macs post sale, in newer ones it's a little more cumbersome, and generally will void the warranty. Since you'll need to disassemble it completely to get to the hard drive. Also the newer SSDs Apple is using, are hard to come by in the seocndary market in larger sizes, and can be very expensive.

The original 2008 and the 2009 MacBook Airs had the option for either SSD or HDD. The 2010 through current models are only available with SSDs. They are upgradeable. The following is one source: http://eshop.macsales.com/shop/ssd/owc/macbook-air

128 is the size of the storage available and SSD is the type.

To find out what model and year your computer is go to the Apple menu and select About this Mac. On about the 3rd line of text in the dialog you’ll see something like: MacBook Air (13 inch, 2012)