Fraudulent tags are a broad way of tagging clicks that is used for clicking patterns that indicate click fraud. These are usually reserved for:
repetitive clicks from the same IP address
repetitive INVALID clicks from the same IP address
clicks from various threats (proxies, blacklisted IPs, etc)
clicks where unique device cannot be determined and user behavior cannot be recorded
clicks from bots
How to tag clicks as fraudulent?
You can opt-in to tag clicks as "fraudulent" in your Advanced Rules. If you are using Simple Rules some clicks will be automatically tagged as "fraudulent".
Learn more about click tags here.