TLS stands for Transport Layer Security and it is a protocol which helps protect your emails by encrypting the connection from the sender to recipient (client to server). Encryption makes snooping of your emails much harder while they travel on their way to your recipients. 

Encryption in transit is important so that your emails are read only by the intended recipients and not on their way there. 

When two servers connect to each other they negotiate an encryption method and use it to secure the communication channel. If negotiation fails or it is not possible then the servers may decide to send the emails unencrypted which means that they are sent in clear text and anyone on the network would be able to read them. Alternatively if negotiation fails or it is not possible the sending server may decide to not send the email.