What the Dollar Sign Does
Understand how $ locks a column or row inside a reference.
Why References Move
When you copy a formula, the spreadsheet usually shifts the cell references to match the new spot. Drag =A1+B1 down one row and it becomes =A2+B2 automatically.
This is normally exactly what you want. But sometimes you need a reference to stay put no matter where you copy it. That is where the dollar sign comes in.
Meet the Dollar Sign
The dollar sign ($) is a tiny lock you place inside a cell reference. It tells the spreadsheet: do not let this part change when the formula is copied.
A reference has two parts: the column letter and the row number. You can lock either one, both, or neither.