Campaigning simply means doing something to put across a point and get something changed.
There is scepticism about whether campaigning can actually achieve anything. You only need to take a quick dip into a history book to see the ways in which campaigns have made massive impacts. Of course there are the big law changing ones like the civil rights movement and the feminist movement. But smaller moves like passing an ethical foods motion through your student union general meeting can make a strong political statement to the rest of the world and spark off the possibility of social change. 
Campaigning can include anything from letter writing and lobbying, to demonstrations and direct action, to encouraging people to cycle and eat vegan food... basically any action intended to bring about change. Whatever your opinion, issue or idea it's your involvement that's the important thing. So go out there, get active and be the change that you want to see in the world.