"I forged a concept which is relatively simple and possibly unique in theology, and that is, the irrational is the primordial stratum of the universe."
- Philip K. Dick

The glossary of terms at the back of Dune states that a solido is a three dimensional image using the 360 degree image imprinted on a shigawire reel, adding that "Ixian solido projectors are commonly considered the best" - essential information for those of you shopping today on Ebay.

Here's a larger image of the projection itself:

(Factory Crawler from 'Dune' by Frank Herbert)

Robert Silverberg gives a detailed description of this idea in his 1956 story The Rivals:

Michael Karren held the solido slide slightly to one side, tipped it up and down to see which was the proper way of inserting it, and slid it into the projector. Instantly the image of his wife Helen and their two young sons took form at the end of his living room, glowing and life-like in three dimensions. He could almost smell once again the sharp sea tang that had been in the air, and as he looked at the solido image he found himself recapturing every pleasant moment of that glorious vacation...

“You’re very proud of this,” Jennerton said. “You think the mere ownership of a fancy solido-projector is an occasion for pride!”

“Of course I’m proud of it,” Karren retorted. “Not many people own a solidoprojector, and I’m proud to be able to demonstrate one in my own home.”