There are two types of cell references: relative and absolute. Relative and absolute references behave differently when copied and filled to other cells. Relative references change when a formula is copied to another cell. Absolute references remain the same no matter where they are copied.
By default, all cell references are relative references. When copied across multiple cells, they change based on the relative position of rows and columns. For example, if you copy the formula =A1+B1 from row 1 to row 2, the formula will become =A2+B2. Relative references are especially convenient whenever you need to repeat the same calculation across multiple rows or columns.
However, there may be a time when you don't want a cell reference to change when copied to other cells. Unlike relative references, absolute references do not change when copied or filled. You can use an absolute reference to keep a row and/or column constant. An absolute reference is designated in a formula by the addition of a dollar sign ($). It can precede the column reference, the row reference, or both.
In addition, Excel allows you to refer to any cell on any worksheet, which can be especially helpful if you want to reference a specific value from one worksheet to another. To do this, you'll simply need to begin the cell reference with the worksheet name followed by an exclamation point (!). For example, if you wanted to reference cell A1 on Sheet1, its cell reference would be Sheet1!A1. If you rename your worksheet, Excel will automatically adjust the cell references to the new worksheet name.