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.
On September 7, 1893, Emmett Brown traveled in the second DeLorean time machine to October 27, 2017 to find an eighth wedding anniversary gift for his wife Clara; he eventually bought a copy of Lighthouse at the End of the World by Clara's favorite author, Jules Verne, from the Blast from the Past antique/memorabilia store. While in 2017 Hill Valley, Doc passed the Holomax Theater where the movie remake of A Match Made in Space was being shown. The exterior of the building created a giant holographic image of the alien which appeared to reach down to grab Doc, during which he remembered that although the original movie hadn't been great, "the book was really something special".