Aperture is, far and away, the hardest leg of the exposure triangle to understand.
The aperture is the size of the opening in the lens. Only light that goes through that opening can hit the sensor. You can see the aperture variation on a large camera in manual mode—it is same way as the iris in your eye that gets smaller or larger.
Some people are confused by this concept: they think that making the aperture smaller will constrict the field of view, much as cupping your hands in front of your eyes constricts what you see. This is not the way it works. To fully understand why is, unfortunately, beyond the scope of this manuscript. Suffice it to say that it does NOT constrict the field of view; it only reduces the amount of light that goes into making that view. Think of the brightness in a north-facing room (in the Northern Hemisphere). A room with a large window will be brighter than a room with a small window.