I would only add that while $P$1 and M49 are both
ways of referencing a cell, $P$1 is call an "absolute"
reference, whereas M49 is a "relative" reference.
The difference arises when you copy the formula.
For example, suppose the formula above is in N49,
and you copy it to N50. M49 will change to M50, but
$P$1 will remain the same.