What's the difference between może and chyba?

może vs chyba

there is some overlap, the differences are (I think - non-native speaker here) is that chyba is more of a (not firm) opinion, estimation while może is just about possibility

może = maybe, possibly

chyba = probably, i think so (sort of), i'm inclined to think so, i hypothesize

this. You might expect this meaning since może is (also) the 3rd p. sg. of móc - which both means potrafić, mieć możliwość - someone is able to do something - and mieć pozwolenie - someone is allowed to do something - but also wyrażenie przypuszczenia - expression of presumption - and the latter meaning overlaps with chyba. Examples from Swan in increasing degree of certainy:

Może Janek będzie chciał ci towarzyszyć. Maybe Janek will want to accompany you.
Chyba Janek będzie chciał ci towarzyszyć. I suppose Janek will want to accompany you.
Prawdopodobnie Janek będzie chciał ci towarzyszyć. Probably Janek will want to accompany you.

chyba has additionally a meaning of unless:
Przyjadę, chyba żebym zachorował. I'll come unless I get sick. This cannot be expressed with może. On the other side, może can be used for expressions that carry no doubt at all:

Może pływać w głębokiej wodzie, a bateria wytrzymuje 10 dni. It can swim in deep water and the battery lasts 10 days.
innpoland.pl/137317,moze-plywac-w-glebokiej-wodzie-a-bateria-wytrzymuje-10-dni-polska-firma-wypuscila-supertelefon-za-250-zl
Here, może has a totally distinct meaning from chyba, because there is no uncertainy.

(the usual cavaet applies: non-native speaker here too, take everything with a grain of salt).

Actually that's 'chyba że' (conditional particles will attach themselves to the że)