Mixed Cell Reference

A mixed cell reference is either an absolute column and relative row or absolute row and relative column.
When you add the $ before the column letter you create an absolute column or before the row number you create an absolute row.

Example
$A1 is absolute for column A and relative for row 1.
A$1 is absolute for row 1 and relative for column A.

If you copy or fill the formula across rows or down columns the relative references adjust and the absolute ones don’t adjust.