What happens to a formula when you copy it is the single most confusing, and most useful, Excel behavior for a beginner.
=A1+B1 copied one row down automatically becomes =A2+B2 — Excel shifts the reference to match, which is usually exactly what's wanted when applying the same calculation down a whole column.
Putting $ before the column letter and row number, like $D$1, locks that reference so copying the formula never changes it — essential when every row needs to use the same one fixed cell, like a tax rate.
=B2*$D$1 (row 2)
=B3*$D$1 (row 3, copied down — $D$1 never moves)
=B4*$D$1 (row 4, still $D$1)It's also possible to lock only the column or only the row: $A1 locks column A but lets the row shift when copied down; A$1 locks row 1 but lets the column shift when copied across. Genuinely useful when building a table where one axis should stay fixed and the other shouldn't.