It means “verified” but only to the extent of “verified a phone number”, per the info provided when clicking on Blue checkmark account.

Of course, in addition to being meaningless, that’s also false in the case of the involuntary Blue accounts. But the absence of verification is a bigger deal with the involuntary gold checks.

Is that a "real" problem, though? Like does that blue checkmark actually change how you use twitter? Have all the historic "verified" accounts become invalidated and fake based on the blue checkmark being tied to the account?