Ideally, we should be able to touch the roof of our mouths with the tip of our tongues, with our mouths fully open.
Some people with crooked teeth may also be tongue-tied, and the two things are related.
The little vertical white line (visible in the pictures on the left below) is a membrane under our tongue called our lingual frenum, which pulls the tongue down to the floor of our mouth. In some people, this membrane is attached farther forward on the bottom of our tongue than typical, restricting the upward and/or forward range of motion of the tongue. This restricted range of motion reduces the pressure our tongue exerts on the inside of our teeth, and if the cheeks and lips are pushing back normally, the teeth get pressed closer together causing them to crowd and/or the upper jaw to develop narrower than it would if the tongue had more upward mobility causing a cross-bite (narrow palate) to develop.
Releasing the tongue-tie increases the tongue's range of motion to better balance the muscle pressure in our mouths to keep our teeth straighter after orthodontic treatment. It can also improve abnormal growth patterns in growing kids if treated early enough. Some tongue ties are significant enough in newborns that they may not be able to nurse, or in older kids they can cause lisping when speaking.