Use the dollar sign to lock cell references in a formula
Lock both column and row: $A$1
Lock only the column: $A1
Lock only the row: A$1
Press F4 after selecting a cell reference in the formula to toggle reference types
On Mac, use Fn + F4 or Command + T depending on Excel version
Example: =A1*$B$1
Example: =SUM($A$1:$A$10)
Use absolute references when copying formulas and keeping specific cells fixed