When you connect to the internet using a VPN your connection is encrypted, meaning criminals cant track you meaning they'll move onto someone else. Your data is scrambled is called a cypher, and there is a key (or logic) that allows you to decypher the message so that it makes sense again.
The highest encryption standard known as AES (Advanced Encryption Standard) 256-bit and it is used by most recommended VPN providers like AVG. What does 256-bit mean? It's the size (or complexity) of the cypher used in the encryption. The bigger it is, the more possibilities there are, and the harder it is to guess the key. With 256-bit encryption, there are more combinations than there are stars in the universe. In short 'weapons grade' encryption used by large corporations, banks, and military around the world.