Opposition is something that goes against or disagrees with something or someone else. Just about any political view has opposition.
Chances are you know the word opposite: this means something similar. The opposition to something goes in the opposite direction. There is always opposition to raising taxes in this county. The Republicans are the opposition of the Democrats (and vice versa). If you're holding a meeting, you could say, "Is there any opposition to this idea?" In war, the enemy is the opposition, and in a debate, the other speaker is the opposition. Whenever there's disagreement or confrontation, there's opposition.