In its broadest sense, the head of a piece of music is its main theme, particularly in jazz, where the term takes on a more specific set of innovation.
No one piece of written music defines what the "head" of many jazz tunes really is, but a boilerplate jazz chart, which is often only a page long in large print, will tell you: as well as more general information such as The form is an even more general and abstract concept dealing with the theoretical context in which the actual music is being played: the chord progression, its sections and other miscellaneous events such as kicks or time changes are all important information that the performers must keep track of.
There are many heads that are considered such a part of standard jazz repertoire that professional players are expected to know them by memory and be able to perform them in a variety of ways on the spot.
On the unfortunate occasion this may be due to confusion about "where" the top of the form actually is if the musicians get off-sync with one another, or a frantic attempt to regain composure and finish the performance, as playing the head to end a piece is default jam session protocol.
Jazz musicians are frequently called upon to play a series of songs in short order with no planning, either at jam sessions or impromptu gigs.