Material Picker

The Material Picker is used to pick Materials directly from the 3D View.

Using the ALT key with the tool active, and with a selection of objects, you can use it to assign materials too.

Menu

The button to call the tool is availabe in the Shading workspace by default.

Preferences

In the addon preferences, you can determine, if it should appear in other workspaces too.

Using Material Picker

Check out these two tweets.