What’s the difference between over and under exposure and why is that important?
Over exposure results in loss of detail in your print (not revealing image). You're rinsing out the image, and parts of it won't rinse out.
Under exposure results in loss of stencil or emulsion can chemically lock to the screen. When you're rinsing out the screen and sections other than the image wash out.
Easiest way to find out if your stencil is properly exposed is to use a 21-Step Calculator. You want to be able to rinse out the image to step seven. If it stops rinsing out before step seven, the screen is over-exposed. If it rinses past step seven, it's under-exposed.