Myofunctional trainers are silicone appliances which, when placed in the mouth, cover the upper and lower jaws at the same time and make the patient bite on the appliance in a particular way. In this way they stimulate the correct growth and development of the jaws. If the patient is at a suitable growing age and has a bite problem caused by the way the jaws have developed, the trainer encourages growth in the right direction. It is a relatively soft appliance and corrects the jaws not so much through pressure as by holding them in a better position and waiting for growth to do the rest of the work.
This answer is for general information. Your own case needs an examination by an orthodontist.