How do I stop a cell reference from changing when I copy a formula to other cells?

By default, a cell reference like B2 is relative: if you copy a formula containing B2 down one row, it becomes B3, then B4, and so on, shifting with the formula. That is usually what you want, but not when a formula needs to always point at the same fixed cell, like a tax rate or exchange rate stored in one place. Adding $ before the column letter, the row number, or both (as in $D$1) locks that part so it does not shift when copied.

$D$1 locks both the column and the row, which is the most common case: a single fixed cell used across an entire column of formulas. $D1 locks only the column (the row still shifts), and D$1 locks only the row (the column still shifts), which matters for formulas copied both across and down, like in a multiplication table.