End to end encryption (E2EE) encrypts your message throughout its whole journey between two end-points. It stays encrypted while traveling through intermediate servers and neither the service provider, nor your ISP or any third party can access it.
Without E2EE, your message is encrypted once it reaches a mid-point server that decrypts it. Thus, an entity controlling these servers (e.g. an ISP) might see your messages. However, if you use a VPN, this type of connection is much safer as a VPN service encrypts your traffic and changes your IP. Then your traffic can pass all the mid-points safely and privately.