End-to-end encryption means that your messages and files are encrypted in every step, not just on the way to a server. When data is encrypted end-to-end, only the sender and the intended recipients can access it. Meanwhile, the service providers themselves can’t access your data, even if they wanted to.
With Peerio, when you write a message and attach a file, your computer encrypts, or “locks”, your message with your private key before sending it over the network. It may be on our network, but it’s still locked, even from us. Only your intended recipients will be capable of decrypting, or ‘unlocking’ your message or files.