They are a type of film in which the viewer is able to feel all the sensations felt by the protagonist through the use of advanced technology such as a "scent organ", pneumatics and an electric field.
In a major scene of the book, the character John, from the low-tech and "savage" Malpais Reservation, goes to see a feelie called Three Weeks in a Helicopter.
The real-life venue for Three Weeks in a Helicopter, the Alhambra Music Hall, was ultimately demolished in 1936 to make way for a theatre, reflecting Huxley's fears that mass-produced entertainment would replace art.
[1] Huxley's idea of feelies was a reaction to the recent emergence of sound films (or "talkies"), such as The Jazz Singer (1927), which was controversial at the time and saw a number of detractors.
[1] In the 1950s, the advent of widescreen technologies, such as CinemaScope and Cinerama, provoked anxious predictions in the press that it would lead to the development of feelies as movies became increasingly immersive.