Okay so the more complex answer is that tjuncts are anywhere a func detail brush touches a world brush, or another func detail brush. Too many of these tjuncts and you get the error you have.
One of the most common places a lot of these are caused are where stairs meet their trim or the wall. An easy fix if you have trim is to make the trim itself func_lod or func_brush, which would stop a tjunct from being made for each step.
Another common place is where a bunch of support beams all meet, like say a central beam. Turning that beam into one of the above mentioned entities can help reduce them.
Mind you with the above fix, your are adding to the entity/ edict count, which is another limitation you have to keep in mind.
Another way to fix a lot is turning some of your more detailed brushwork into models.
After that, reducing detailing certainly doesn't hurt.
You can monitor your tjuncts with the waterindices section of your compile log.
Tjuncts are a big pain in the ass and often pop up with late stage very detailed maps. So congrats if that's where you got!