Copying a formula moves every reference with it. That breaks when all the rows should use the same cell, such as a tax rate in B1: in the next row =B4*B1 becomes =B5*B2, and B2 isn't the rate.
A dollar sign locks part of a reference:
$B$1 locks the column and the row. It never moves. This is an absolute reference.B$1 locks only the row; $B1 locks only the column. These are mixed references.
You don't have to type the dollar signs. While editing, put the cursor in or right after a reference and press F4. Each press cycles B1, $B$1, B$1, $B1. On a Mac laptop press Fn+F4; Excel for Mac also uses Cmd+T, but your browser keeps that one for new tabs.
A good habit: when a formula points at a single setting cell (a rate, a target, a total), lock it.