You may have heard of something called a “trusted third party” before while using a web browser. This is usually used in reference to security certificates. This might seem confusing at first but it’s actually very simple.
Alice and Bob have come up with a nearly fool proof way to stop people from eavesdropping on them. The last thing they need to do is get Alice’s public key to Bob through Mallory without Mallory modifying the key or replacing it.
To do this they use Trent. Trent has started his own business of securely transmitting public keys for people.
The idea is that everyone already has Trent’s public key. Alice simply needs to securely send her public key over to Trent and he’ll sign it with his public key and send it out to anyone who asks for it. This way Bob doesn’t need to have Alice’s public key ahead of time just to be able to create an encrypted connection with her.