While editing a formula, this shortcut toggles cell references from relative to absolute, to partially absolute, back to relative again:
A1 --> $A$1 --> A$1-- > $A1-- > A1
It's much faster and easier than typing $ characters manually.
To convert an existing formula, enter cell edit mode, place the cursor in or next to the reference you'd like to convert, then use the shortcut.
Note: in Excel 2016 for the Mac, you can also use fn + F4.
Quick, clean, and to the point.