It's an "opera" in the sense that it requires a very large and well-equipped stage to perform (as opera houses usually area), but also in the tradition of co-creator Robert Wilson's slow-theater "silent operas", which were seven, twelve, even twenty-four hour productions that illustrated a central theme in a very, very lateral way. You can see a "produced" example of one of his most famous early pieces here: https://www.youtube.com/watch?v=4t-NtrLzgkE On film/video it loses some of its power but that's the general gist of it - very, very slow, with dreamlike surprises (the window suddenly frosting over, the lamp over the sink bending) and a sense of both mysterious surprise and unutterably ancient ritual.
And Philip Glass, who came from the same late-60s-early-70s New York punky school of art as Wilson (see also: Meredith Monk, Talking Heads, Laurie Anderson), is, of course, the famous minimalist composer, whose music focuses on repetition and shifting patterns of tones and harmonies. At the time he took on Einstein on the Beach, Glass had recently finished an almost three-and-a-half hour work called "Music in 12 Parts", which he considered a summation of all that he'd learned up until then. You can hear the first part here: https://www.youtube.com/watch?v=j7fcHnR7UF0 Again, generally representative, and you can probably see why Glass and Wilson would go well together.
The third major contributor was Christopher Knowles, a deeply autistic boy Robert Wilson had previously collaborated with on several shows, including A Letter For Queen Victoria, which went to Broadway in 1974 (a very different time, surely). You can see examples of his “typings” and poems on Google, and are typically both obsessive and almost non-euclidian, exploding and looping in abrupt ways, visiting and revisiting patterns and points of reference. For Einstein on the Beach, Wilson sat down and asked Knowles what he knew about Albert Einstein, who Glass and Wilson had settled on as their subject. Knowles went into his room and typed up a twelve-part manuscript that had essentially nothing to do with Einstein at all, and acts more like a kind of wild, kaleidoscopic reflection of American culture, such as the “I Feel The Earth Move” monologue, which you can read here http://nicolas.sceaux.free.fr/einstein/text_trial2.html and which is spoken onstage at a breathlessly fast pace with no pauses at all.
The opera itself is performed in its four-and-a-half-hour entirety without an intermission, and theatergoers are allowed to exit and enter the auditorium as they like. It’s a nice idea in theory but in practice it’s actually kind of annoying, because getting up and down for people who have to pee or want to stretch their legs breaks the concentration that’s essential to the show. Beyond that, it’s also annoying to have to look away, because things are constantly happening. Wilson’s general mode is to pack the stage with objects and people, all moving on their own planes and intents, in converging and diverging patterns. Aided by his design and lighting, as well as the texts from Knowles and others, and Glass’s hypnotic, pulsing music, it becomes a piece of contemplation. You can see an assortment of (handheld-bootleg) clips here: https://www.youtube.com/watch?v=8evRrDmdyQM
A full taping of the production was recently made and will hopefully be released soon, but it’s easy to find if you put your eyepatch on.