To start, I will give you the scenario of a balloon filled with gas molecules.
Gas molecules inside a volume (in this instance, a balloon) move around freely and constantly within this given space. As they move, they collide with one another frequently, as well as with the surface of any enclosure there may be. The force of impact against the surface of the enclosure is what is known as gas pressure. The more collisions that there are, the greater the measure of gas pressure. This means that as the size of the enclosure decreases, the pressure will ultimately increase, because the gas molecules are more likely to hit the surface of the enclosure as they move about because the area in which they can move has decreased.
In the case of the balloon, the internal pressure resides within the balloon, which are represented as gas molecules moving within and hitting the inner surface of the balloon, exerting pressure within it. The external pressure resides outside of the balloon, represented by gas (air) on the outside, ...
The solution discusses what would happen if Boyle's Law focused on internal pressure in 725 words.