When used as a stand-alone term, 'cricothyroid' refers to the cricothyroid muscle in the larynx. This muscle stretches the vocal folds by pulling together the sides of the cricoid cartilage and the thyroid cartilage that are opposite the vocal folds.
To envision this, imagine that the thyroid is a board standing on a ring (the ring being the cricoid). The vocal folds stretch from the top of the back side of the board to one side of the ring. The cricothyroid stretches from the top of the front side of the board to the other side of the ring. Tensing either muscle will tilt the door towards that muscle and pull on the opposing muscle, stretching it tighter. Pulling on the vocal folds makes them thinner and tighter, raising the pitch of the voice. This is the primary mode of changing the pitch of your voice.
If this seems overly complicated, keep in mind that if you simply tensed the vocal folds (i.e. the thyroarytenoid internus), you would shorten and thicken the vocal folds, resulting in lower pitch. For a full range of pitch it is necessary to be able to both tense and stretch the vocal folds.
A Little More Detail For the Medically Minded:
The inferior cornu ('lower horns') of the thyroid sit atop facets on either side of the cricoid lamina. These points of articulation are known as the cricothyroid joints. The cricothyroid muscle has two heads, the pars recta and the pars oblique; both originate at the lateral surface of the cricoid cartilage. The pars recta inserts at the inferior border of the thyroid, and the pars oblique inserts at the anterior surface of the inferior cornu where it joins the main body of the thyroid. When contracted, these muscles pull the front of the thyroid cartilage downwards, pivoting over the cricothyroid joint. The vocal folds stretch from the thyroid to the arytenoid cartilages (which rest on the cricoid); thus, when the thyroid rocks forward the vocal folds are stretched tighter.
The cricothyroid's antagonist is the thyroarytenoid (vocal fold), which originates at the lamina of thyroid cartilage just below the thyroid notch and attaches to the arytenoid cartilages. These two muscles rock the thyroid cartilage back and forth constantly during normal speech, and together they are the primary of controllers of pitch in the human voice.
The cricothyroid is innervated by the external branch of the superior laryngeal nerve. It is the only laryngeal muscle not innervated by the recurrent laryngeal nerve; thus, one result of unilateral recurrent laryngeal nerve injury is obligate falsetto (i.e., a very high pitched voice), although hoarseness and aphonia are more common outcomes.
Part of Tem42's Praxis II Quest