Regular expressions (also called RE or regexes) are a tiny, specialized programming language which can be used to do pattern matching and extract parts of a string.
If you have an address column that is formatted like this:
123 Cherry Road, Mobile, AL 36571
9 Main St, New York, NY 10022
3050 Hollywood Blvd, Los Angeles, CA 90210
Then you could extract the state abbreviation by searching for two uppercase letters followed by a space and five digits, like this
[A-Z] means any uppercase letter, and \d means any digit. The parentheses specify which part of the matched string to extract into a new column.