To immobilize something is to make it impossible for it to move. You can immobilize a battery-operated toy by taking the battery out.
You could immobilize your brother's car by letting the air out of his tires, or immobilize your little sister by making her so scared of an imaginary monster in the bathroom that she can't move a muscle. In both examples, someone or something is prevented from moving. To immobilize is to make immobile, or motionless, from the Latin root immobilis, which means both "immovable," and also "hard-hearted."