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.
Use cAn i MiNe thIS bLOCk? if you are on Fabric!