If understood in general terms, the prefix по- is attached to some of the specific verb, a smooth transition from indefinite to something more specific.

Prefix по- may have various meanings (may add some new meanings to a verb). Here are the main ones: 1) doing something little by little, not intensively: по+отстать = поотстать (to lag behind slightly) 2) an action conducted many times or by many people: по + вынести = повынести (to take/carry everything out, one by one) 3) to perform an action during some (short) time: по+курить = покурить (to take a cigarette, to smoke) 4) To start an action defined in the main verb: по+бежать = побежать (to run, to start running) 5) To complete an action, to have something done: по+благодарить = поблагодарить (to thank someone), по+завтракать =позавтракать (to have a breakfast, and have the breakfast eaten). Source: Russian Grammar, http://rusgram.narod.ru/850-884.html

It is one of the prefixes in Russian that denote aspect, which is a grammatical category which defines how long an action is done, and other temporal flow variations of actions (like perfect vs, imperfect in English). По- in this case is either the perfective aspect, or the durative aspect (the action occurred for a while/a bit). For example: я ставил=I was putting/I used to put, поставил=I put. Also, я говорил=I talked, я поговорил=I talked for a while.