Glandular (adjective) means of or relating to the glands. The glands are organs in an animal (including human) body which secrete/ manufacture substances for use in the body, or for ejection. For instance, the sweat glands secrete the water which is released as sweat when the body becomes too hot.

In glandular fever, the lymph glands swell (lymph is a colourless fluid containing white blood cells.) This causes the patient to become lethargic.