When you create a Peerio account, a public key is generated from your username and account key. This is tied to your account, and it's how Peerio is able to verify the identity of you and your contacts when sending Peerio messages, and files.
Because Peerio manages public key exchange, and associates your public key with your username, you can sign-in and communicate with people using only your username. Peerio will even warn you if a user's Peerio ID# has changed since you added them as a contact.
However, there may be times that you want to personally verify that the user who are contacting is who they say they are. You can do this by clicking or tapping on the users avatar to bring up their profile. Call your contact, and ask them to tell you what their Peerio ID# is.
If the Peerio ID# they give you matches what you see in their profile, all is good! If there are discrepancies between what they tell you and what you see, you should inform your contact and immediately cease communications with that Peerio account until you can verify a correct Peerio ID# with your known contact.