Ransomware is a very common type of Malware that stops you using your computer or opening / editing / printing files until you have paid a Ransom.
Ransomware describes how criminals install a program (via a trojan or similar) onto your computer which then Encrypts your files. This means that when you open them they are just random letters, numbers and symbols rather than words, images, videos etc. It often displays a lockscreen (see image) with a message saying you must pay a ransom to regain use of your computer.
The ransom message usually includes instructions for how to pay the fine, which is often by credit card or Bitcoin. Ransom amounts range from less than £50 to several thousand pounds.
If you don't pay the ransom, and can't remove the ransomware then you lose those files. Sometimes when you do pay you still don't get access to the files - they are criminals after all, and there is no guarantee that they will do what they say.
Ransomware examples you may have heard of include: WannaCry, Cryptolocker, Petya and Bad Rabbit.