Definition: An insidious type of malware. The cybercriminal tricks users into "clicking" and the illegal program locks or encrypts the users computer files. You would be prevented from accessing your data, until you pay the ransom being demanded. In theory, when the malicious user is paid the computer owner will be sent the password to unlock the files.
Its Relevance: An effective ransomware attack can render your computer useless. A cyber criminal only has to trick a user to 'click' and the ransomware can be downloaded. You absolutely must completely backup on a device or site that is separate from your computer and be capable of restoring all of it.