Regex stands for "regular expression" and is a way to search for existing patterns within text. It is not language-specific, meaning that it can be used to find patterns within JavaScript, Python, Ruby, C++, etc. These expressions actually don't have to do with programming at all; they can be useful for general find-and-replace searches in a file. For programming, regex is often used for form or input validation.
There are literal and meta characters:
Literal characters are what we see - what is literally written, like "github.com."
Meta characters are ones that do not depict a single, literal character, but a generalized pattern.