What happens when my capacity go low?

if you are talking about battery health percentage....

may experience unexpected shutdowns and performance scaling..

see the related article at the upper left of this page and the one below

iPhone Battery and Performance - Apple Support

Do you mean when you are close to using the available memory on your phone? If that is what you are asking, when you use most of your memory, your phone can't operate well. Apps may not run, processes slow down, the phone may freeze or stop.

Why are you asking?