Ideally, the top back teeth overlap the lower back teeth. Much like a lid to a pot. However, when the top jaw is too narrow for the bottom, a crossbite will occur. A narrow upper jaw is often accompanied by crowding of the upper jaw as well.
This bite is unstable and leads to heavy wear facets on areas of the teeth not designed for heavy forces. Left long term often leads to jaw joint discomfort and asymmetrical lower jaw growth.
Crossbites like these are best treated with young.
Crossbites are best treated when young. Treatment is simple involving an fixed or removable expander over a short period of time.
In adults, minor crossbites can be addressed with a combination of plates, fixed expanders and Braces or Invisalign. Severely narrow cases may require surgical intervention to widen it sufficiently to match your lower jaw.