By default, it shows whether or not you can currently harvest the block, the required harvest level of the block, and the tool type that is effective against the block. Plus, there are many configuration options in the Waila modules configuration screen to tweak it to show exactly what you want it to.
Note: This mod is mostly intended for use with Tinkers' Construct, but the information is still applicable without it.
The idea for the mod was suggested by DraxisWuf.
For similar functionality on Fabric, see the Can I Mine This Block? mod
While holding an Iron Pickaxe (with default settings):
Silk Touchability and Shearability:
Waila configuration screen:
You are welcome to add this mod to any modpack you'd like.