Parathyroid glands are small, brownish-tan glands that are located in the neck, usually around the thyroid gland (see picture below).  In majority of people there are 4 glands (84%), around 3% of people have only 3 glands and 13% have more than 4 glands.

Parathyroid glands control the calcium levels in our blood through a complex feedback loop involving parathyroid hormone (PTH), serum ionized calcium and Vitamin D.

Calcium is needed in our body electrical energy throughout our body, but especially nerves, muscles and heart).  It is also involved in the bone structure.
Parathyroid glands sense the level of calcium in our blood and increase or decrease the level of PTH to keep the calcium levels within certain levels.