Forwarding your keys basically. It's like giving someone you don't know a handshake AND all your keys.
Edit: I don't really know how do say this short and concise, but you should only do this with servers you trust.
* http://rabexc.org/posts/pitfalls-of-ssh-agents
* http://heipei.github.io/2015/02/26/SSH-Agent-Forwarding-cons...
Yes, it's really stupid to enable AgentForwarding.