An outlier is a number that's far away from the rest of your data.
For example, if we look at the numbers 3, 2, 4, 5, 27, and 1, you can likely consider 27 to be an outlier because it's so far away from the rest of the data.
The specific definition of how far something has to be from the rest of the data to be an outlier depends entirely on the situation, but in general: if a number obviously sticks out from the rest, it's probably an outlier.