Depth of field is a term we use for how much we want something in focus when you make a photograph. Sometimes you want a narrow depth of field, which would mean just your subject is in focus. Other times you might want a large depth of field and have everything in focus. This is a decision that you make every time you make a new image.
For a narrow depth of field you use a small aperture setting, usually the number is low f/2, f/4 or maybe f/5.6. For a large depth of field (everything in focus) you would use a large f/stop such as f/11, f/16 or f/22. The larger the number the larger the area in focus, the smaller the number the smaller the area in focus will be. It is that simple. You can get into the tech side and I could give you all sorts of reasons and examples of how it works, but you don’t need to know how it works, just that it does.
This can get a little more complicated depending on the lens you use. The concept is the same, on a wide angle lens, the smaller the f/stop the smaller your depth of field, the larger the f/stop the larger your depth of field. The same with a telephoto lens. The one trick is that with a wide angle lens the depth of field is larger even with the small f/stop, so you don’t see the limited depth of field as easily as with a telephoto lens.
Here is an example of a narrow depth of field, the fist figure in the line is in focus the rest are out of focus.
In this example, this one shows a large depth of field, and all the figures are in focus.
So with a narrow depth of field, you are isolating your subject and with a large depth of field usually your subject is the whole frame.
So experiment with your depth of field, try some narrow to see how you can isolate your subject and try some with a large depth of field. It is a great tool to have and once you understand it, your images will improve.