A hologram was a futuristic method of displaying an image, using light to form a 3D object.
In 1991, Marty McFly made use of a holographic tutor when learning about the civil war. However, 1991 did not appear to be technologically advanced enough for holograms to be commonplace, so it is likely that Emmett Brown brought it back from the 21st century to aid Marty's studies.
In 2015, a holographic shark was used in the promotion of the movie Jaws 19 at Hill Valley's Holomax Theater. After Marty was 'swallowed' by the holographic image of the movie's title character, commented to himself that the shark "still looked fake", perhaps meaning that the technology was in its early days and not yet very realistic. Goldie Wilson III had a holobillboard advertising his hover conversion company, Goldie Wilson Hover Conversion Systems.