Ransomware: the name says a bit of what it entails. Once the malware is installed on your computer, it will start finding files (i.e. pictures, documents — basically anything that could be of value to you) and will start to encrypt it. That means you cannot open your file without the appropriate key from the attacker. They usually will demand payment in the form of BitCoin for the decryption key. If you decide to pay the ransom,  you may or may not get your files back.

What is the best way to prevent ransomware?

Essentially, it makes its way to your network via inadvertent installations of malicious software.  This can be tough to know what and what not to click on. (Wasatch Security Awareness can help with this).

Always back up your data. Always back up your data. Always back up your data.

If you have a copy of your data that is current, there is no need to pay anyone a ransom to get your files back (which is not always a guarantee anyway).