It is commonplace in everyday computer use to utilise the "find" function to locate a match in a body of text. Asking the computer to "find" text is would typically mean providing literal characters for the computer to search in a body of text to highlight results (e.g. "cat").
Regular Expressions, often referred to as Regex, is a powerful tool that allows the user to step outside the boundaries of literal character search and opens up the option for users to additional tools to create more complex matching.
When thinking about Regex, it is best to understand them as string of specialised characters that describes a pattern of characters for the system to find and match in a given string or body of text.
Regex will feel cryptic to anyone who hasn't been exposed to them, but with some guidance and practice, you'll understand enough to start to make use of them.