A footprint is a description of how a device should be soldered/connected on a PCB. Note that different devices may share the same physical package, and therefore share the same footprint. For example, SO14 (small outline, 14 pins) is a very common package. Many different 14-pin devices use the same package.
Most PCB design tools come with a library of footprints. However, there are many proprietary footprints that are not included in a built-in library. Fortunately, most tools allow the additional of more custom footprints.