It describes the amount of your photo (how far away from you) that is in clear focus.
From Wikipedia:
The DOF is determined by the subject distance (that is, the distance to the plane that is perfectly in focus), the lens focal length, and the lens f-number (relative aperture). Except at close-up distances, DOF is approximately determined by the subject magnification and the lens f-number. For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DOF; decreasing f-number decreases DOF.
Me again:
In other words, on any given lens, increasing your aperture number (reducing the diameter) will give you a bigger range of what can be in focus, and decreasing the aperture number (increasing the diameter) will give you a smaller range. However this will also affect the amount of light coming in to the camera, so you may have to adjust your shutter speed to compensate.
All other things being equal, you can have more things in focus at f22 than you can at f3. But rarely are all other things equal. Changing aperture means you might have to change your shutter speed since everything is ultimately connected. Switching to a longer lens with a larger focal length might also be recommended in some situations where you need more depth of field.
