Every block has baked in whether it can only be harvested with a tool. And if so, which tool class and which harvest level of the tool is required. For example, redstone ore requires a pickaxe with at least the mining level 2, while glowstone can be harvested with an empty hand. On the other hand, every tool has baked in the information, what kind of tools it is and if used as such a tool, which harvest level it has. For instance, an iron pickaxe is a pickaxe - and only a pickaxe - with mining level 2, where a paxel is a combination of a pickaxe, an axe, and a shovel. This mod allows you to modify the tool class and the harvest levels of each class for all tools. You can also modify the required tool class and the harvest level for blocks. This allows mod pack makers to define a progression for tools.

See the Wiki.

• WAILA Harvestability is lying to you. If you define a block overwrite, which changes a block to require a tool that didn't require one before, WAILA still shows, that the block is harvestable. Don't worry though, you can break the block.
• Block Overwrites can't be done on a per metadata basis, so only one overwrite for all metadata within one block. A future update will solve that. Block Overwrites are now only done per metadata. Will add support for per block overwrites again.

Yes, you can use this shitty mod in any mod pack you like. This includes redistribution as a part of a modpack. If you make a shitload of money, please send some beer and pizza.