F4 Absolute Reference and Repeat
When you need Absolute References (dollar signs) in your formulas press F4. Succeeding presses of the F4 key return all the permutations of absolute and relative reference: $A$1, A$1, $A1, A1.
To make a single cell reference in your formula absolute, click somewhere on the reference then press F4. For a range reference or a series of references, drag across the references first before pressing F4.
F4 has a double life, when you are in Ready mode (i.e. not editing your formulas) F4means Repeat. Say you have the tedious job of going through a worksheet and deleting some of the rows, delete the first one as you usually do and then select the next row to be deleted and press F4 to repeat the row deletion.