If you listen to good guitarists you'll note that they often put in a riff or a run of notes when changing from one chord to the next. It's usually very short and rarely complicated - a hammer on here, a pick off there, maybe a bit of a scale. But for all its simplicity it adds a great deal of flair to your playing. A fill is nothing more than an interesting transition from one chord to another. Often what keeps a good beginner or intermediate from making the next step forward is an inability to incorporate fills into his or her playing.
The simplest fills are best summed up by that wonderful cliché “the shortest distance between two points is a straight line.” You're on a C chord (in a song conveniently in the key of C major). The next chord in the song is a G. All you have to do is walk up (or down) the C major scale to G. Pretty easy, isn't it?