The toolClass parameter for "normal" items refers to the type of vanilla tool it will act like. When applied to a custom block, it sets the type of tool that can break it (easily), as well as what tool type will have an added bonus with Efficiency, assuming it does not have 0 for its hardness.

The tool classes available are the same as the vanilla tools. These are: -Pickaxe -Axe -Shovel -Hoe -Sword

To use this parameter, simply add this to your item or block's javascript file, for whichever tool you wish, and whichever metadata it should apply to:

toolClass[0] = "pickaxe";