The Quick Definition popup shows you where and how a project symbol (class, method/function, field, etc) is defined:
View definition of a symbol at caret
Select a symbol and press Ctrl+Shift+I or choose.
You can open a definition in the Find tool window. To do so, click the icon in the top-right corner of the popup and click Open in Find Tool Window.
To open the source code of the definition for editing and close the popup, click the icon and then click Edit Source (F4).