Excel has three addressing modes: Relative, Absolute and Mixed.
Absolute addresses will reference the same cell no matter where the reference is moved or copied in a worksheet. For example, when you copy the formula =$A$3 from B3 to C5, it will still read =$A$3.
A Relative address will reference the same cell in relation to any cell where it is copied. For example cell B3 is one cell to the right of A3, so when you copy the formula =A3 from B3 to C5, it will now read =B5.
Mixed mode addressing contains one relative coordinate and one absolute coordinate, for example $A3 or A$3. Mixed mode addressing is extremely useful for quickly generating reports using NL and NF functions.