A $ before the column and/or row freezes that part of a reference. Use absolute references when a formula must always point to the same cell as you fill it down or across.
Do it with AI insteadType the reference
Click the cell in your formula, e.g. =A2*C1.
Press F4 to lock it
With the cursor on C1, press F4 to make it $C$1.
Fill the formula
Drag down or across — the locked reference stays put.
=A2*$C$1It locks the column ($A), row (A$1), or both ($A$1) so the reference doesn't shift when you copy the formula.