It's a kind of 'font' for music. Soundfonts contain samples from an instrument, which are saved for specific range of musical notes inside the file. For example, if you record every single key of a piano, you can build those into a soundfont-file, which makes it possible to use the instrument in any way digitally. Some instruments are easy to record, some are really detail work.
Here you can hear or download some of those I made if you are interested in music production or have the possibilty to use them.