Typically your upper teeth protrude out over your lower teeth.
This makes it so that when you close your teeth you see mainly see your upper teeth.
In some cases the lower teeth will cover the upper teeth when you close.
This is known as an under bite.
Under bites can come in two forms.
#1 is a problem with the tilt of the teeth.
If the lower teeth are flared out or the top teeth are pushed in then an under bite may occur.
This problem can usually be corrected very easily with orthodontics.
#2 is a skeletal problem.
This means that the lower jaw grew to far out or the upper jaw didn't grow
out far enough or both.
This kind of under bite can also be corrected with orthodontics, but
in severe cases may require surgery to fix.
Best thing to do is to get your child to an orthodontist if you suspect that trouble.