Four choices
$A$1 : both column and row do not change while copying
$A1 : row changes while copying down, no change while copying across
A$1 : column changes while copying across, no change while copying down
A1 : both change while copying across or down
That is exactly what it does - keeps the row or column constant depending
upon where it is inserted. If before the column, then the column is
constant. Please read up on absolute and relative references in Help menu