The median is the middle value in a group of values. For example, the median of the values "1, 2, 3, 4, 5, 6, 7" is 4, because it's the value in the middle of the group of seven numbers. There are three values before and three numbers after the 4.
If a set of values has odd numbers in it, the median is always the middle value. However, if a set of values has an even number of values, the median is the average of the two middle values. For example, the median of the values "1, 2, 3, 4, 5, 6, 7, 8" is (4 + 5) / 2, which equals 4.5. The values 4 and 5 are the two middle values, and the average of those two values is 4.5.
In Microsoft Excel, the function below, when placed in a cell, fills with the median of the values in cells A1 through A7.