The adenoids are normal lymph nodes (“glands”) located up the back of the throat, between the throat and nose (see Diagram — Upper Respiratory Tract). Just like the tonsils, they can become swollen, usually due to common childhood infections. If swollen enough, they can interfere with air passage, & give a sense of nasal congestion. Worse, they can cause Sleep Apnea.
The adenoids can’t be seen by a normal clinical exam; they require a special instrument for endoscopy used by Ear-Nose-Throat (ENT) specialists (Otorhinolaryngologists). That’s one reason we refer patients with chronic undiagnosed nasal congestion. If truly necessary, adenoids (and tonsils) can be removed by surgery. But it has to be truly necessary.