'mean' is another word for average.
so if you want to find the mean of a group of numbers you add them all up and then divide by how many there are.
If the scores (numbers) are 2,3,8,5,9 then
mean = (2+3+8+5+9) / 5 = 27/5 = 5.4
The symbol for mean is an x with a bar (line) over the top.