Slash Chords are chords where the bassnote changes. You may have seen something like this: C C/b Am For this example you play a chord of C for one bar, then lower the C bassnote to a B while keeping the rest of the previous C chord in your fingers. For the last chord you play a Am. If you obseve the movement of the bassnotes you will notice they fall down step by step, they form a little melody or movement on their own.
Slash chords are great for adding some spice to your chords.
The opposite of using Slash chords is using pedal notes: Keep one note (bassnote) the same while you change the rest of the chord. Take this idea a bit further and you get to drones: One note (or a few) which serve as harmony for the rest of your melody (or improvisation). Drones are used often in Indian Music. Some African music also uses this kind of idea.