The 'median' is the middle value in a data set when the set is arranged in order from smallest to largest. For example in 3, 4, 5, 6, 7, the median value is 5. For sets containing an even amount of numbers, the median is halfway between the central pair.
The median is another way of analysing a data set, and along with the mean and mode is another form of an 'average'. In some situations it would be more appropriate, such as when the highest and lowest values are expected to be large and inaccurate.