In their online Transparency Report, Google made it abundantly clear which email providers support encryption, and which do not. They based this on the 425 million active Gmail users and found about half of email traffic is not private. The Transparency Report allows users to search by region to find out if their email provider is using encryption.
Encryption is basically an encoded message, meaning no one can read it except the sender and the recipient. Think of encrypted email as sending a locked box, where only the sender and receiver have the key. In unencrypted emails, others have the key too. Unencrypted emails mean people can snoop your inbox, and considering how privacy obsessed we are becoming, that is extremely undesirable.
For encryption to work as intended and protect your digital activity, both the recipient and sender much have the encryption protocol, called Transport Layer Security (TLS). If you are using Gmail, which is encrypted, but emailing a non-encrypted email provider, the entire email chain becomes unencrypted. The new End-to-End plug-in will change that, by encrypting it the entire way, no matter who you are emailing with.