CTM stands for Complete The Monument, and originated from quite a long time ago.
They are Minecraft maps where you have to complete a monument as a goal. It started with having to obtain 16 wool blocks in their respective areas and placing them in a monument, but is now declined in various ways. It doesn’t have to be items, nor does it need to have a solid monument to complete. There are even some CTMs made in other games; At this point it is simply a genre of doing something with a goal to accomplish.
But the most common ones stay in a fairly standard format with global rules. You’ll be playing in Survival Mode, with no cheating and variable rules. The rest is usually fair game, using what you’re given the way you want.
The more modern maps have a tendency to become closer to action adventure with customized gear and enemies, capitalizing on datapacks for even more elaborate mechanics. There are as many styles as there are mapmakers and the diversity makes it truly a unique genre.
The best place to find them is the CTM repository. Maintained and containing most of all the CTM maps ever made.
The document I maintain with all the maps I reviewed.
There is also a discord server for the CTM community to share and talk about CTM maps.