Make It Fancy or Make It Crisp
In addition to light, aperture affects your "depth-of-field", which is a fancy way to say how much stuff is in focus.
Depth of field is like taking a giant slice of cake out of the space in front of you. Stuff in that slice is sharp.
A big hole means the slice of cake is shorter. Stuff behind and in front of your focus point will be blurry.
A small hole makes the slice of cake very long, so almost everything is in focus.
In a portrait, you might want a blurry background, so you would use a big hole (large aperture, small number). In a landscape, you may not, so you would use a small hole (small aperture, big number).