The mean is a type of average of a set of numbers.
The mean is the total of the numbers divided by how many numbers there are.
Try starting with a specific example.
Group A is composed of people aged 2, 7, 11, 14 and 16 (average 10)
Group B is composed of people aged 5, 11, 12, 13 and 19 (average 12)
Investigate what happens to the averages when different members move from one group to the other.