s- na- po- are just some of the prefix called "aspect markers". In English "I lost my key" and "I've lost my key" have slightly different meaning, the latter usually implying that you haven't found your key yet. This nuance is called "aspect", in Russian "vid": "ja terjal svoj kljuch" vs "ja poterjal svoj kljuch". So, aspect is the...*"shape"*...of an action: sometimes you want to say that something has taken place, or that it has had an effect, or it has lasted for a while, or maybe it was carried out in different directions. These are all aspects, but russian only has two main families: imperfective and perfective (well, verbs of motion also have a bydirectional aspect) and each of them covers several aspects. This is why in dictionaries you find aspectual couples of verbs, and the perfective is often marked by one of those prefixes. Prefixes also can carry the meaning of "conceptual direction", giving the chance to derive a lot of different verbs, but this is another story.

Hi! you have to read about Perfect and Imperfect verbal aspect. It will help you a lot in this subject! f.e. you can read this: http://www.russianforeveryone.com/Rufe/Lessons/Course1/Grammar/GramUnit13/GramUnit13_1.htm