As a prefix in the beginning of verbs it can usually mean some limited Duration of action. For example: покататься, погулять, покрасить rather than кататься, гулять, красить (more abstract meaning in terms of the spent time). As a preposition it usually means literally "on the", in physical terms "above". But in English it can be be translated as "in" or "through". Идти по парку. And if you meet words like по-своему, по-русски this prefix means "the way to do something" with describing word what the way it is. По-своему = своим способом. По-русски = как делают русские. It's absolutely not exhaustive translation but you need to show examples because of huge amount meanings for this word in Russian.