What is that the thyroid actually does?

I never fully understood my thyroid problems, though I was diagnosed at the age of seven when I was found to have "early development." Basically from the way the doctor had put it, she put me under the impression that I was growing faster than I should have been,and it wasn’t normal at my age, et cetera. But now, she has upped the dosage on my medication to one of those spiffy violet colored pills, about thirty milligrams higher than my old medication. She said my thyroid levels [after a blood workup] had been "…dangerously low." What does that mean? I am utterly confused, by what she means, and what this means to my health… What does the thyroid actually do; what did my doctor mean? Any help would be appreciated, and I welcome long explanatory answers divulging the thyroids inner (and outer) workings.
*What is it that the thyroid actually does?

Hehe,erm…sorry.