Having only a UDP connection available is not enough. Though suitable for transmitting data, we want to be able to reliably send other information, such as routing and session key information to somebody.
TCP is a better alternative, because it already contains protection against information being lost, unlike UDP.
So we establish two connections. One for the encrypted VPN data, and one for other information, the meta-data. Hence, we call the second connection the meta-connection. We can now be sure that the meta-information doesn’t get lost on the way to another computer.
Like with any communication, we must have a protocol, so that everybody knows what everything stands for, and how she should react. Because we have two connections, we also have two protocols. The protocol used for the UDP data is the “data-protocol,” the other one is the “meta-protocol.”
The reason we don’t use TCP for both protocols is that UDP is much better for encapsulation, even while it is less reliable. The real problem is that when TCP would be used to encapsulate a TCP stream that’s on the private network, for every packet sent there would be three ACKs sent instead of just one. Furthermore, if there would be a timeout, both TCP streams would sense the timeout, and both would start re-sending packets.