A myofunctional trainer is a bulky silicone appliance that you bite on with your upper and lower teeth at the same time. It is used as an additional aid for correcting harmful habits in childhood (incorrect swallowing, mouth breathing, thumb sucking), together with exercises to break the habit. The trainer can help guide the teeth in the right direction while they are growing. As a retention aid, the trainer can be used to maintain a corrected deep bite, open bite, Class II (distal) bite or Class III (mesial) bite. It does not hold individual teeth well, and unless it is combined with another retainer, slight crowding may reappear in teeth that were corrected during orthodontic treatment.
This answer is for general information. Your own case needs an examination by an orthodontist.