Focus is a function of a camera's lens and the current aperture setting. An object that is in focus in crisp and clear, while one that is out of focus will appear blurry. Photographers have many ways to manipulate and adjust focus. Some prefer to manually focus a shot using the focus ring. Point-and-shoot camera users often rely on autofocus, which is a system that lets a camera's sensors detect the subject's distance as a motor automatically adjusts the focus. Autofocus is very handy, but has its limits if you're shooting several subjects at various distances from the camera, or subjects that are moving toward or away from the camera.
Some advanced cameras have continuous autofocus -- it can actually track moving subjects and keep them in focus no matter where they go. If one of these cameras isn't in your budget, you can manually set your focus for a certain distance and time your photos when the subject reaches the right place. You can even "game" the autofocus by forcing it to focus on some object at the proper distance, then taking the actual photo of an entirely different subject that moves into that same distance.