An ear plug is something placed in the ear to restrict someone's ability to hear. In a BDSM context, this will often be as part of a sensory deprivation scene and the ear plugs may be supplementary to an isolation hood.
Ear plugs are never 100% effective; even if the ear is well filled, sound can travel through the bones of the skull. A suitable hood can help to reduce such sound. A better technique is to make the victim wear headphones that play white sound or something else that can drown out other sounds.