RE: Excel
It's where you put a $ in before a reference. This forces Excel to look at the very same cell no matter how far you drag out your formula
so if you were to put in C1 the following fomula: =A1+B1 and drag the formula down from C1 thru C100 the formula in those dragged out cells would be, for instance =A25+B25
but if you put =$A$1+B1 and drag it out, the formula in cell C25 would be =$A$1+B25