Ok, say you had in cell A3 a formula =A1*A2, alright? If you copied cell A3 into cell B3, it would automatically shift to =B1*B2. Following me so far?
But if you wrote in A3 a formula =$A$1*A2, then when you copy and paste A3 into B3, C3, and so on, instead of getting =B1*B2, =C1*C2, etc, youll get =$A$1*B2, =$A$1*C2, etc. The dollar signs hold the number instead of shifting it, which sounds like what you want.