It's effectively a version number. SpaceX have been quite inconsistent in how they've named their rocket versions, with Falcon 9 v1.0, 1.1, 1.2, Full Thrust, another upgrade that didn't get a name (informally called Fuller Thrust), and Block 5. Perhaps Elon is just messing with us. Effectively Block 5 is supposed to be the last iteration of the Falcon 9, with a slight performance increase and a lot of improvements to reusability, mostly from what was learned from the landed cores.
I suspect at this point that Elon's entire goal in naming things these days is to troll people. The naming of his autonomous barges after sentient spacecraft from Iain Banks novels is quite clever, and his announcement of 'The Boring Company' should really put to rest any questions of whether he's messing with us with naming things. I find it quite funny, personally.
Also notice the pronunciation Space-sex, and Tesla models s,3,X (he wanted a model E, but settled for 3 after Ford complained). He is clearly trolling.
