Hello!

Using the $ sign will make the cell an absolute reference:

$a$1 - A is the column and 1 is the row. If you put a dollar sign before "a", regardless of how you move the formula (copy-paste/drag), "a" will continue to be "a", and if you put $ in front of "1", "1" will always be "1".

if $a1 - "a" will stay the same regardless, and "1" will change

if a$1 - "a" will change and "1" will stay the same

For more tips on excel and financial models, you can check this out. Hope this helps!