Roles apply for 4v4 or 5v5 matches. Sometimes for 3v3 but that is rare.

Suicide position that has on left and right another enemy team player. Even if you are skilled it is hard to defend against 2 players attacking you simultaneously. 

Eco left and right there are allied players. Your job is to develop massive economy and feed players that are fighting.

Fighting on one side there is allied player on the other there is enemy. Your job is to develop fleet and take enemy player out of the game.

PS This is really short description but there are threads on this forum written by skilled players dedicated to roles and what you should do.