You’ve probably noticed that while copying formulas, the cell references in the original formula automatically update to reflect the new formula position. For instance, if you copy the original formula down one cell, the new formula row reference numbers would be increased by one. So, in this case, a cell that original read: =B1+C1, would read, after copied: =B2+C2.
This is called a Relative Cell Reference. All formulas contain relative cell references by default.