Normally the upper teeth overlap the lower ones, and the upper dental arch is the wider of the two. We have a crossbite when one tooth or a group of teeth have swapped position – a lower tooth sits further out than an upper one. It can affect the front or the side teeth and, like any bite problem, it can be at the level of the teeth or of the jaws. A crossbite caused by the jaws most often means that the upper jaw is narrow and needs to be widened. This is usually done with an appliance other than braces or aligners, which may be a removable or a fixed expander. Once the narrow jaw has been widened, treatment can continue with braces or aligners to align the teeth.
When only one or a few teeth are in crossbite and the jaws are in a normal relationship, the problem can be corrected with braces or aligners, and sometimes by wearing elastics hooked onto the teeth involved – a cross elastic.
This answer is for general information. Your own case needs an examination by an orthodontist.