A scam is a dishonest scheme (often illegal) designed to trick someone into giving money. Scammers may contact you by telephone, email, text, WhatsApp, WeChat or another social media platform.
They will often pretend to be from an official organisation (the Home Office/UKVI, the police, the Courts of Justice, an Embassy), and may appear genuine as they hold information about you (passport details, address etc). They may say that there's a problem with your visa, that you have not attended a court hearing, or are suspected of a crime.
As a result they will demand money, often calling it a fine, to rectify the problem. They often claim that if you don’t pay quickly, the fine will increase, you will be deported, or your visa will be cancelled.