Depth of field is about how much of a photo you want to be in focus.
In most landscapes, photographers tend to use a big depth of field so that everything from the flower in the front to the mountain in the background is in focus. On the other hand, many portraits have a small depth of field. Photographers will often blur the background while the subject is kept perfectly sharp. To have a small depth of field, you use a small aperture number like 3.5. To have a big depth of field, you use a big aperture number like 22.
Take a look at the trees behind our model Nyarai. This is a shot with a small depth of field.
Just like shutter speed, aperture affects the amount of light in your photo. We can worry about that later. Just start thinking about big (22) or small (3.5) depths of field, and how you could use them.