1. This does not mean it will not mesh but can be the reason it does not.
2. Clicking on each item shows what the problem is and is usually obvious.
3. Generally it is a good idea that the geometry represents something that can physically happen.
a: two parts cannot physically occupy the same space.
b: a single part cannot have geometry folded inside itself (the same XYZ position both inside and outside the part at the same time for example)
4. CAD can generate item 3 especially from inexperienced users and from ignoring or turning off error messages while modeling.
5. The solution is to fix/correct the CAD model and there are about as many ways to do this as there are possible bad geometries. Without seeing your model and being able to click through the warnings no specific help can be given.