The outlier is the number in a set of data that doesn’t match the rest.
For example if you’re talking about a test and the grades look like this
58%, 85%, 87%, 86%, 90%, 89%
The outlier would be 58% since it isn’t close to the rest and would make the overall average drop.
With the 58%, the average score on the test would be 82.5%
But without it, the average score would be 87.4%
so it’s just a number that’s far higher or far lower than the rest of the data