The Opposition is an aspect between two planets that are 180 degrees apart (give or take a few degrees of orb). This signifies a stressful, polarising energy between the two planets, which can also be motivating. It can also provide a complementary balancing of planetary energies.
The areas of life represented by the two planets can get caught in a "tug of war" where they fight for dominance. However, when they work well together, they complement and balance each other out, each contributing benefits and qualities that the other lacks.
Sometimes there is a dynamic of projection, where you identify with one planet and the energy of the other planet seems to come from outside of yourself (like another person or a situation). This can often be experienced as a struggle beween yourself and the other person, and this may be due to your inability to own or embrace the qualities of the projected planet. As you learn to own and identify the opposite planet's characteristics within yourself, these kinds of struggles usually decrease or become lighter.