OK, as you say, you have a private/public pair key so I can send you encrypted e-mail, because I know your public key.
But how do I know your public key? Obviously, because you told me what it was, and if you told me over a secure channel - for example you wrote it on a piece of paper and handed it to me in private - then that's fine.
But if you emailed it to me over an insecure channel how can I know that Eve didn't intercept your email and replace your key with hers? Then Eve could read my encrypted email, because I'd have encrypted it with her key.
This is a real problem because if we've only got the Internet as our communications channel then by definition it's not secure to use for exchanging keys. It can't be, because we haven't exchanged keys yet.
One way round this is to involve a third party who we both trust. Let's call him Bob.
Bob has a lot of secure channels to people. He has one to me and so I know his public key. He has one to you, and Bob knows your public key.
What Bob can do is take your public key, and encrypt the message "Lance's public key is 18348273847473436" with his private key. He then gives that encrypted message to you. Bob has signed your key.
Now, if you want to send me your public key, you just send me the signed key over the insecure channel. I know it came from you, not Eve, because I can decrypt it with Bob's public key, and I trust Bob.
So, we've turned the problem of "Graham and Lance need a secure channel between them" into the problem of "Graham needs a secure channel to Bob" and "Lance needs a secure channel to Bob".
It doesn't sound like we've made things better, until you realise that now instead of worrying about arranging secure channels to D.W. and Lucas and Schroeder and Thomas and Rory and Graham and everyone else on the Internet, you just need to worry about getting a secure channel to Bob, once.