A mixed reference is either an absolute row and relative column or absolute column and relative row.
You add the $ before the column letter to create an absolute column or before the row number to create an absolute row.
For 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.