If you place the cursor over any name in your code, the Workbench
will try to find a usage message. If it can it will display
this in a popup. For example, placing the cursor over
Module in the code shows the usage message for Module.
The usage messages for system symbols are found in the Mathematica layout, so if you do not get a usage message, you should check your preference settings. If you have usage messages defined for your own functions, then these will also be displayed.
The popup shows other useful information, such as attribute settings, and if the symbol is exported from a package it will show context information.