Lock a reference with dollar signs

Relative references are the default, and here they are the problem. Every row needs to multiply by the one tax rate in B1, but copy =B4*B1 downwards and the second half walks down the sheet with it, pointing at B2, then B3, then nothing.

A dollar sign freezes the part that follows it. $B$1 means column B, row 1, always. Put it in before you copy and the rate stays put while the net amount moves.

Pressing F4 while the cursor is on a reference cycles it through B1, $B$1, B$1 and $B1, which is faster than typing the dollars.