Think of gustar as "to please," but without the sexual connotation.

Me gusta means "it/he/she pleases me" — though we would actually say "I like it/her/him."

Me gustas means "you please me."

The verb is conjugated based on who/what is doing the pleasing.

As noted below, if you mean "like" in a romantic sense, then you'd use different phrasing. And if you mean like in a casual way, as in "I like that guy," you'd probably say something like "me cae bien" (depending on region).