A range is the term employed by Excel to define a group of cells.
For instance, B1:D5 represents a group of 15 cells (B1, B2, B3, B4, B5, C1, C2, C3, C4, C5, D1, D2, D3, D4, D5).
Many functions accept ranges as argument. For instance: SUM(B1:D5) returns the sum of the values contained in the 15 cells above.
Ranges can be named. To discover the way to accomplish that, read the following pdf file.