To add on to what u/latexi95 said, the work units you get are hours of processing and you are usually given around 4-7 days to complete it. Once completed and on wifi, your PC will send the completed units back to the project automatically.

Astroids@home was something I used to crunch on my old android phone in 2015 or so, that hopefully can give you an idea of how simple the work units can be.

BOINC is actually pretty smart, and it will check your hardware before it sends anything, so if there are GPU focused work units vs CPU focused one it will check before they are sent.