Selection pressure is one of those important ideas that human intuition isn't designed for. The idea helps visualize how selection (natural or otherwise) works. I usually picture it as a force pushing the evolution of a species in a particular direction. An even richer visualization is the pressure of a potter's hands shaping a piece of clay; the potter representing the selection pressures of the environment, and the clay being the traits of the species.
Selection pressures act on both predators and prey in rainforests to encourage them to find ways of blending into the foliage. Those that are poorly camouflaged are selected against and filtered out of the gene pool.
Similarly selection pressures shape behaviours of people around us. For example there are certain subjects politicians will not voice a controversial opinion on. The subjects can vary by culture but will almost always include things like insulting national pride, criticizing the military, etc. Selection pressure acts against politicians foolish enough to make these mistakes.