Slime Blocks are translucent Blocks that the Player can bounce on.
Slime Blocks are used in lots of self-propeling Redstone contraptions. They are also used as trampolines to break a Mob or Player's fall. They are used for easy storage of Slimeballs.
9 Slimeballs => 1 Slime Block
1 Slime Block => 9 Slimeballs