Its called a frenulum.
It is the little piece of tissue that connects your lip to your gums (see below).
Its also the little piece of tissue that connects your
tongue to the floor of your mouth (see below).
Why are these pictures important?
Because your dentist may tell you that your child, or you, needs a frenectomy.
That is where the doctor will cut the little piece of tissue in half.
This is done in cases where the frenulum is getting in the way.
Either it is in between the teeth causing a gap or
in is tying the tongue to the floor of the mouth and thus hampering speech.
Its just good to educate yourself on what things are and why certain
procedures are going to be done before actually getting them done.
And there is always the option of piercing your frenulum.
But.... I'm not a big fan of that.