A mean is what we typically think of as an "average." It's the number whose value is in the center of the given values.
A mean is found by adding up all the numbers and then dividing the sum by the number of numbers:
For example, if you have the numbers "2, 3, 7", you would add them together to get 12, and then divide by 3 to get 4:
Means are very useful, especially when you are looking to forecast something where outliers will be averaged out and not matter individually.
For example, if you get $200 per day during the weekdays, and $0 per day on the weekends, you can average these out and know that you can spend an average of $71.42 per day. It doesn't matter that you don't earn money on the weekends because the money from the weekdays exceeding the average will balance them out.