TCP stands for Transmission Control Protocol. Basically a means of sending traffic over the Internet with some built-in measures to ensure that traffic can get to its destination. If anything goes wrong during transmission, the protocol has some means to try to find a solution (send the packet of information again or try an alternative route or such). TCP Meltdown occurs when you stack one transmission protocol on top of another, like what happens when an OpenVPN TCP tunnel is transporting TCP traffic inside it. The underlying layer may detect a problem and attempt to compensate, and the layer above it then overcompensates because of that, and this overcompensation causes delays and problems with the transfer of data. That's the layman's version of it that is easy to explain and understand. We therefore instead recommend that you use UDP, which has no transmission control, and on top of that send your TCP traffic as usual, so that there's only one layer of transmission control, and the problem can be avoided.
Some people mistakenly believe that TCP is the best protocol to ensure the best reliability and performance for sending traffic over the Internet. This is the exception.
If you want to learn more there's a good article here on an external website: Why TCP Over TCP Is A Bad Idea