You can fix formula contents (cells/ranges) using the F4 key which cycles through the options

$A$1 = Absolute reference it will keep A1 no matter where you drag
A$1 = Absolute Row reference - it will keep Row 1 if you drag down but will change column if you drag across
$A1 - Absolute Column reference - it will keep column A if you drag down or across, but will update row when you drag down