The mean is most affected by outliers. The mean is the average of all the numbers, so if one number is really big or really small, it can change a lot.
For example, let's look at these 10 numbers: 2, 3, 4, 4, 5, 5, 5, 6, 7, and 100.
To find the mean, you add all the numbers:
2 + 3 + 4 + 4 + 5 + 5 + 5 + 6 + 7 + 100 = 141.
Then you divide by 10 (because there are 10 numbers):
141 ÷ 10 = 14.1.
Now, if we just took out the 100, the numbers are 2, 3, 4, 4, 5, 5, 5, 6, 7.
Adding these gives:
2 + 3 + 4 + 4 + 5 + 5 + 5 + 6 + 7 = 41.
Then divide by 9 (there are 9 numbers):
41 ÷ 9 = about 4.56.
See? The mean changed a lot because of that one big number!
The median and mode are not changed much by just one outlier. So the mean is the most affected.