What does a battery rub with better performance (notebook) mean?

First of all luck in the round,

You can, I think that is the case with every PC, to set or determine the performance of the battery. The funny thing is, nothing changes in the brightness. Then what does "better performance" mean? That the PC works faster and the battery drains faster or what? Do I understand that correctly or am I wrong?

The CPU is normally limited in battery mode. With the "mode" better performance it is not limited or if not a little bit like in "normal" mode.

I can only tell you how it is with me:

the battery drains faster
but the PC is faster and smoother

Ok then I'm still right

So the PC becomes more fluid if I set it to the stop

Yes.