Basically, they're used in financial transfers to identify banks (or other financial institutions).
A Swift code is the most common format of a Bank Identifier Code (BIC) - so a Swift code is a BIC.
The Swift code consists of either 8 or 11 characters (8 if the code is for the primary office of the institution, and 11 in all other cases).
The first four characters (which are only ever letters) are the bank code, the next two characters are the country code, the two characters after that are location code, and the (optional) last three characters are the branch code.
You can look up Swift codes here: https://www.swiftcodes.info/australia/