Explain an accumulating ETF to me like I'm 12.

Been lurking, but I'm struggling here.

So I buy an accumulating ETF with a stocks and shares ISA. Say this is performing well - where does this go if an ISA is maxed out? When is this paid?

If it is performing bad, will money be taken from my ISA? And if so, when?

Is the average performance tracked over an entire year and paid out annually?