Average is when you take into account all extremes and find a middle ground.
It is easier to explain in the world of math. If I have $10 and you have $20 the we have an average of $15 between the two of us.
If you are a 10 and your friend is an 8 then together you average 9.
C is considered an average grade but most people want A's and can achieve them if they try.
Average work will not get you fired but it will not get you promoted quickly either.
Average athletes will not win championships and average fighters should probably avoid fights.
To sum it up average puts you in the middle. With average it is safe to assume half are better and half are worse, but this is not always the case because their can be a strong curve involved.