Learn Polish: O co chodzi? Dlaczego płaczesz? - What's the matter? Why are you crying?

The phrase o co chodzi literally means "about what does it walk?" It uses the verb chodzić (to walk), but when combined with o, the phrase chodzić o forms a very common idiom meaning "to be about" or "to be the matter". It is the most natural way in Polish to ask what is going on or what someone means.

Could I use "po co" instead of "dlaczego" to ask why the person is crying?

No, because dlaczego and po co have different meanings. Dlaczego asks for the reason or cause of an action, which makes sense here since crying is an emotional reaction. Po co asks about the purpose or goal ("in order to do what?"). Since people don't usually cry to achieve a specific practical goal, dlaczego is the only correct choice here.

The infinitive for crying is "płakać". Where does the "cz" in "płaczesz" come from?

The verb płakać has a consonant mutation in the present tense where the k changes to cz. This happens in many Polish verbs ending in -kać. As a result, the "you" form becomes płaczesz instead of just adding an ending to the basic infinitive root.