What is Outlier?
An outlier is a value that is far from the rest of the data, which may be a genuine extreme or an error.
Outlier explained
Outliers matter because they distort averages and totals. Before removing one, find out what it is: a typo (an order of 10,000 instead of 100), a test record, or a real but unusual customer.
Common ways to flag them are a z-score beyond about 3 standard deviations, or a value more than 1.5 times the interquartile range outside the quartiles. These are conventions, not laws.
Example
One order for 250,000 among thousands of orders under 500 is an outlier that could be a bulk purchase or a data-entry error.