A sagittal section is a cross-section
of the head, parallel to the sagittal suture
on top of the skull.
Specifically, it is a diagram used in phonetics, showing the position of the tongue and other organs in and around the mouth, with the nose and lips to the left and the throat to the right. Depending on what sounds need to be shown, the whole pharynx down as far as the larynx may be shown. The nasal cavity is drawn in, but obviously nothing above that is used for speech (after it's left the brain), so that's as far as it goes.
Sagittal sections are now sketch diagrams, but were originally based on x-ray photographs of people actually speaking, or at least carefully making the sounds, in the early 1900s. Of course the still anatomy had been known for centuries, but it is notoriously difficult even for a trained phonetician to be quite sure what their tongue is doing on a particularly tricky sound. X-ray photography enabled the position of vowels, for example, where the tongue lies in or near the centre of the mouth, to be charted with accuracy.
Familiarity with these diagrams is essential to linguistics students. Any book on phonetics will show numerous of these cut-away pictures to explain the differences between sounds.
I have found a rather cute thing on the Web: an interactive sagittal section. By clicking radio buttons for different features you instantaneously get both a redrawn picture, and the IPA symbol for the resultant sound. See