According to the KMT pressure is directly proportional to the number of collisions between particles.
So in the container a bunch of gas particles are constantly bouncing off each other, and they are also bouncing off the sides/ walls of that container. The force of these collisions is called pressure. It's like if a bunch of people started hitting a wall. They would be exerting a certain force against that wall. For gases, pressure is exerted by the tons of little particles moving really fast and slamming blindly against those walls.