In 1931, professional golfer Gene Sarazen designed the first sand wedge by building up solder on the sole of a niblick (comparable to a 9 iron today). By doing so, he created what is now called bounce.
Bounce, also known as sole inversion, is now associated with all wedges and allows the club to glide, or bounce, through the sand or rough rather than dig into it. Generally, lob and pitching wedges have lower bounce than sand and gap wedges, though there is a lot of variance between clubmakers.
On a golf course with soft turf and fluffy sand, wedges with more bounce help prevent the club from digging into the ground and causing a chunked, or fat, shot. On the other hand, if you play on a course with firmer sand in the bunkers, use wedges with less bounce.
Bounce is measured in degrees in relation to the ground. Pitching wedges generally have between 2-5 degrees of bounce, lob wedges 0-10 degrees of bounce. Higher bounce clubs include sand wedges with between 10-16 degrees of bounce and gap wedges have 5-12 degrees of bounce.
Changing the loft of your club also changes the angle of the sole and, therefore, the bounce. Decreasing the loft could eliminate the bounce and even create a "scoop" or "dig" sole, which would promote a fat shot.