Validation rules for text allow you to check if text has a certain length or contains certain characters or sequences of characters.
Access provides an expression language that allows you to express such rules.
This expression language contains elements of regular expressions. Regular expressions, or regex in short, is a language that is used in computer programming to match text against certain criteria.
Expressions like the ones you will see on this page are not used just in validation rules.
They are used in other parts of Access as well, most importantly in SQL queries to match records against certain criteria.