Your capacitor is the energy your ship has to run modules. It continually regenerates but some modules are very power hungry and will use more than your ship makes. The fitting window gives you an idea of this by telling you how long your capacitor will last when all your modules are on.
For some things, being “cap stable” is important so you can operate indefinitely, but other times combat is brief or you won’t use all your modules all the time so it doesn’t matter. People often obsess about it too much in my opinion, and the best is to just test out your fit and see if a drained capacitor really is a problem. It usually isn’t, at least for what I do.
There is more info here: