When PODER is combined with SER it takes on the meaning, maybe, might be, could be.
Pode ser. Maybe.
Quem pode ser? Who could it be?
Você pode ganhar tudo! You might win everything!
PODER is usually conjugated in the singular forms because it’s most common that you are saying it might be, he/she/ might be ETC.
But you can use poder in any conjugation. SER is always in the infinitive for this expression though.
Eles podem ser corretos. They might be right!