In insert and command mode, <Ctrl-r> followed by a sequence inserts an object.
Insert mode usage mainly deals with manipulating registers. However, in command mode, <Ctrl-r><Ctrl-*> inserts special objects for certain *:
  • F: Current filename under cursor
  • P: Current filename with expanded path under cursor
  • W: Word under cursor
  • A: WORD under cursor
  • L: Line under cursor
For more, :help c-^r.

Insert word under cursor

Very useful with :Ag, or when doing find/replace.
:Ag <Ctrl-r><Ctrl-w>