The string you provided is a designed to test for or exploit vulnerabilities in a database's search or filtering "feature."
: This is the core of the exploit. It instructs the database to combine the results of the legitimate query with the results of a new, malicious one.
In short, this is not a legitimate feature of a software product, but rather a used to see if a website is properly "sanitizing" the text that users type into search bars or forms.
: This attempts to "break out" of the original SQL query. It provides a keyword, then uses a single quote and a closing parenthesis to trick the database into thinking the intended command has ended.
: This is likely a unique "fingerprint" or tag used by security researchers or automated scanning tools to identify if the injected code was successfully executed in the results.