Method of averaging is a way to get an average of different numbers. An average is a number that shows the middle of all the numbers. So if we have three numbers, 4, 8 and 20, then the average is 11. That's because the numbers around 11 (4 and 8) are the same distance from 11 as the number above it (20).
To get an average, you can add all the numbers together and then divide by how many numbers you have. So for 4, 8 and 20, you would add them together to get 32, and then divide by 3 (because there are 3 numbers) to get 11. That's an example of the method of averaging.