It's regarded by many as the best prog album ever, but that of course is widely debated in the prog community, because everything is widely debated in the prog community, even what constitutes the prog community itself.
I believe the story goes:
Tull had released Aqualung and, just for the hell of it, Ian Anderson gave "Titles" to Side One and Side Two. People started looking for common themes between the songs (and of course they found them, even if none of them were intentional). The most obvious is that the character Aqualung is mentioned in the song "Cross-Eyed Mary". People started calling it a concept album, and Anderson was actually kinda miffed because it most definitely was not a concept album, and he should know since he wrote it.
So he said "Okay, you want a concept album? This is a concept album." And he started taking all the songs he'd been working on for the next album and seeing if he could string them together. And then, what the hell, see if he could make it all fit together, with recurring musical and lyrical themes and variations, the works. And because he still wasn't really serious about it, threw in some spoken word and an orchestral section, because those kinds of things show up in prog, you know. That was Thick as a Brick.
And again, people ate it up. He said "No, no, no, this wasn't supposed to even be serious!" and started writing A Passion Play, which was meant to be a concept album, an actual passion play. The rest is (oft-disputed) history.