I'd tell you to look it up
but I know you won't so I'll summarize.. ;)
hypo- is a prefix meaning "under, beneath, below, lower, less than, etc."
a thyroid is a pretty little butterfly-shaped gland located at the front, lower part of your neck that produces hormones which affect just about every part of your body, including but not limited to: your brain, heart, muscles, skin.
"The thyroid controls how your body's cells use energy from food, a process called metabolism. Among other things, your metabolism affects your body's temperature, your heartbeat, and how well you burn calories. If you don't have enough thyroid hormone, your body processes slow down. That means your body makes less energy, and your metabolism becomes sluggish."
If you'd like to know more, I'll include some more links below. ;)