An entry for the Encyclopedia Galactica appears in every story of Foundation, so you'd think it'd play a more prominent role in the novel. But nope. It only really shows up in the second story, "The Encyclopedists." Still, it's a super important symbol, setting up the themes of scientific knowledge and change that'll appear and reappear in the entire Foundation series. (They're so important we have two sections of our Themes section dedicated to "Science" and "Time," so don't forget to check those out.)
Okay. Here we go.
The Encyclopedia Galactica is a reference guide of all the knowledge assumed by the Galactic Empire. The original idea is to have educated men (sorry, ladies) collect and preserve all this knowledge for future generations, so they can rebuild the Empire in 1,000 years rather than 30,000 years.
But wait, there's more. When Seldon appears in the Vault, he reveals the entire Encyclopedia Galactica to have been nothing more than a fraud. He didn't care for one microsecond if the thing was completed or not. The whole point of the Encyclopedia was to create a "small world of scientists surrounded by [the] vast and rapidly expanding reaches of barbarism" (II.7.26).
Wait. What? Why?
Enter the symbolism. Before Seldon's message, the scientists on Foundation thought their sole goal was to gather knowledge to finish the Encyclopedia. They didn't use that knowledge for anything; they simply collected and cataloged old facts, like an avid collector would do with Star Wars action figures or, you know, M&M merchandise.
But Seldon's message showed them that they couldn't just gather knowledge anymore. They had to use it. The point of knowledge isn't to stockpile it in preparation for their appearance on Hoarders but to further their scientific, political, and economic needs and change their situation. As Seldon says, the point was to have the scientists "[face] hard necessity, and that action [be] forced on [them]" (II.7.27). (Though, you'd think he could have also apologized for fifty years of effort being flushed down the toiled.)
So the take-away? The Encyclopedia Galactica is supposed to represent the proper use of knowledge. Knowledge amassed for knowledge's sake is useless. If you only collect knowledge then nothing can change, and as Hardin warns us, "a worship of the past [is] a deterioration—a stagnation!" (II.5.76).
The knowledge has to actually be used and tested, and that tested information has to be used to actually make things better. In short, things have to change. Only then is knowledge actually worth anything.