Hi,

The word average has many different meanings. For example look at my response from Google when I asked for a definition of the word average. You can also look at what Wikipedia has under average. All the uses carry the idea of the middle or centre or norm of something. We use the word in many different ways in common conversation

The average person has 10 fingers.
An average size of an apple.
The concert I went to last night wasn't outstanding, just average.
On the average my class is well behaved.

One of the meanings is what you see in a mathematics class. If you have a collection of numbers then you add the numbers and divide by the number of numbers in the set. Statisticians and mathematicians want a term for this quantity that isn't confused with the other meanings of the word average so they call this quantity the mean of the set of numbers.

I hope this helps,
Penny