Guitar specific one here.
Inside the necks of many guitars is a rod called a truss rod, and its purpose is to provide reinforcement for the force the strings put on the neck, what we call "relief."
Now, the proper relief for a guitar setup is based namely on the string gauge and tuning you use, because those things are the biggest deciding factors on the tension the strings exert on the neck.
When adjusting the truss rod, we're basically making a very fine adjustment where we want the neck to be almost imperceptibly curved, because if the neck was dead on straight, the strings would just buzz against the frets. So we adjust it to where it looks dead on straight, but it's imperceptibly curved.
Getting this right involves turning the string into a straight edge by fretting the 1st fret and using your elbow to mash down the strings where the neck meets the body, and then using your thumb to gauge the distance between the top of the fret, and the bottom of the string.
There's many ways to visualize it, but I use the gauge of about half the thickness of a credit card as the proper relief. If it's not half the thickness of a credit card, then it needs to be adjusted, and this is a fine adjustment, as as little as 1/8th of a turn can make a big difference. You basically adjust, wait 30 minutes, check again, and you stop when it's right.
Now, some guitars have more stable necks than others, and some you have to adjust more often, and some barely need any adjustment at all, but that's the purpose of the truss rod, and when to make an adjustment. In other words, if the relief is right, leave it alone.
Now, here's where the dumb comes in.
People suggest a truss rod adjustment for any number of guitar related issues, sometimes when it has little or nothing to do with the problem at hand, leading to confusion on what the truss rod is doing, and what the adjustment is doing.
Namely, people suggest a truss rod adjustment when someone says their action is too high. Action is how high the strings sit off the fretboard, and is adjusted after you adjust the truss rod, and you raise or lower the action by raising or lowering the nut and bridge of the guitar. Neck relief barely has anything to do with the action.
In other words, people view the truss rod adjustment as a cure-all for all guitar related problems, and while you do have to adjust the truss rod when setting up a guitar to play and sound its best, it isn't going to fix everything.
Besides that, I've seen people wrench way too hard on truss rods when you really only need a fine adjustment or two. It may provide reinforcement, but it is a delicate piece that can easily break if you're not careful.
So I've been saying that if you don't know what the truss rod does, you're better off leaving it the hell alone, and seek help from a responsible adult.