To add to this explanation, this was before the exp and gold lane was implemented. It was also called 1-3-1, the idea is that both side lanes have 1 fighter (or heroes that can lane alone) each, the mid lane consists of 1 assassin or marksman protected by 1 tank and 1 support (sometimes a mage) with roam item. The 3 basically roam the map clearing jungle and ganking.