Median is the number that is between the lowest and largest number in a set of data or numbers when you list them from smallest number to largest. When there are two middle numbers is between because there are an even number of numbers in the data set, you will find the mean of it.
{1, 3, 5} - 3 is the Median because it is in the middle.
{1, 3, 4, 5} - 3.5 is the median because 3 & 4 are in the middle and the mean of (3 & 4) is 3.5 because 3 + 4 = 7 and 7 / 2 = 3.5