When you create a Peerio account, a public key is generated from your username and secret passphrase. This is tied to your account, and how Peerio is able to verify the identity of you and your contacts when sending Peerio messages, Ghosts, 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 public key and crypto-avatar 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 going to the "contacts" tab, selecting your contact, and finding their avatar and public key in the contact info. Call your contact, and ask them to tell you what their public key is, and to describe their crypto-avatar to you.
This is also possible when viewing a Ghost. To view a senders public key, place your mouse over (or tap) the blue key icon next to the senders name.
If their description matches what you see, 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 public key with your known contact.