The concept originated in the drug counterculture of the 1960s.
Here's a keep-it-simple-stupid answer: A head film is a film that appeals to marijuana users (potheads), and/or users of other psychedelic drugs. The general idea is that these films are best enjoyed while under the influence. They typically feature "trippy" or psychedelic elements.
Many children's movies and other animated fare have historically been re-appropriated as head films by college-age audiences; Disney's Fantasia and Alice in Wonderland are prime examples.
There's also a slew of more "deliberate" head films -- psychedelic art films from the late '60s(ish). A couple popular and representative examples are Invasion of Thunderbolt Pagoda(1968) and Vertige (1969).
Some of the most popular, archetypal head films emerged from the cult/midnight movie and avant-garde film scenes/circuits; further research into those subjects is recommended.
Head - film - was created on 1968-11-06.
The duration of Diamond Head - film - is 1.78 hours.
the head of mrs challfont
Jaws. The first one.
Rebecca De Mornay played the unfortunate Caroline Suzanne whose head ended up in the dryer.
The duration of Head - film - is 1.42 hours.
Machine Head - film - was created in 2000.
Head - film - was created on 1968-11-06.
The duration of Turk's Head - film - is 1.45 hours.
The duration of Head of State - film - is 1.58 hours.
The duration of Machine Head - film - is 1.32 hours.
The duration of Diamond Head - film - is 1.78 hours.
Diamond Head - film - was created on 1963-01-30.
Head Case - film - was created on 2007-09-08.
Turk's Head - film - was created on 2010-03-31.
Head of State - film - was created on 2003-03-28.
The duration of The Case - film - is 1.5 hours.