Actually, this time the security flag was stored in the radio, not the bootloader (for the Evo 3D/Sensation), but the explanation was pretty much correct.
Root is a user/permission that grants a user or application total control over a system. It's a Unix/Linux term. The equivalent on a PC would be Administrator (or really System, but to keep it simple, Administrator). For Android, all you need to think of it as is the Superuser app and the "su" binary (a binary is an executable or command in the form of a file). Superuser can be downloaded from the Market, which is no problem. But it's useless without the "su" binary (file), which needs to be flashed/stored in the /system partition of the phone.
Well if you can't write to /system without root, and you can't get root without writing to /system, you have a bit of a problem.
You see, most phones can be easily rooted with apps like Gingerbreak or SuperOneClickRoot because their bootloaders aren't locked. But HTC likes to add a second layer of protection: S-ON.
See this picture
? It's a picture of the bootloader on the Evo 3D. It was pretty much the same exact thing for the Evo 4G, and is the same for the Sensation. In the top right corner, you see how it says S-ON? It stands for Security ON. S-ON is a flag in the bootloader (or in this case, radio) which disallows any non-system applications from touching the /system partition. What AlpharevX and TeamWin did was develop a tool which, through an exploit, allowed the flag to be changed to S-OFF, thereby allowing us to install a custom recovery (ClockworkMod or twrp), which now allows us to flash things (like the zip to install Superuser, and the su binary).
One last thing I left out is that most apps that require root also need busybox, which can be installed through the app from the Market, "BusyBox Installer", but it's useless without root.
Hope that helps!