What Does The Compound Button Do!!??

hello,

so this will probably seem really obvious to most and i am probably making myself look
like a huge fool but. can someone explain to me what the compound button actually does and how would it benefit me?

making a post about it is probably the wrong way to ask but i cant figure out any other way and i apologise muchly!

P.S. finally figured out how to add pictures!

much love JC!