What does this message mean??

It means exactly what it says. Your phone has crashed at least once because the battery's health has degraded and it is unable to supply peak power.

The CPU has been throttled to prevent it from causing the phone to crash completely by attempting to draw more power than the battery can supply. This is preferable to having your phone simply shut down in the middle of something unexpectedly.

The battery needs to be replaced. make an appointment at the Genius Bar of your local Apple Store or take it to an authorized service provider to have the battery replaced.

The battery health is an estimate based on a performance model. Usually the modeling is good where it should be able to function all the way down to 0% without an auto shutdown. Sometimes the model breaks down (especially with lots of cycles), or the battery diagnostics are corrupted and it's not accurate.

A new battery will more than likely fix your problem.

There's more than one way a battery can fail. Lost capacity is the most common way, but there are others, such as increased internal resistance. That appears to be what has happened to your battery. the battery needs to be replaced. The current Apple price for out-of-warranty battery replacement is $49. They can do it while you wait in an Apple store. If the phone is still in warranty or is covered by AppleCare+ it is free.

deggie wrote:

"The battery diagnostics are corrupted?" Really? I don't see anything that would suggest that here nor have I run into it. Nor will a lot of cycles affect the "model".

I never said that the OP's battery diagnostics had to be corrupted, but that's a possibility with modern battery management systems. I've seen a battery reporting negative capacity. Obviously there's no such thing as negative capacity. The value recorded in the battery's smart battery management system was corrupted when the computer overheated and went into thermal shutdown.

And yes, battery health is estimated using a model since it's impossible to "see" the number of electrons. After 4+ years my iPhone 4S with its original battery would drop its battery percentage 10-20% suddenly. The models/algorithms are getting better, but certainly not perfect. The best way to get an estimate of the battery health is by monitoring the current going in/out. Here's a technical article from Analog Devices:

https://www.analog.com/media/en/technical-documentation/technical-articles/A-Closer-Look-at-State-Of-Charge-and-State-Health-Estimation-Techniques-....pdf

y_p_w said, "Sometimes the model breaks down (especially with lots of cycles), or the battery diagnostics are corrupted."

.

"The battery diagnostics are corrupted?" Really? I don't see anything that would suggest that here nor have I run into it. Nor will a lot of cycles affect the "model".

You do understand what "sometimes" means?