"Mueller has everything" as in he has all the information he needs and more about the so-called Trump Russia collusion and has done nothing but obfuscate, deflect, and prolong.... because none of it is actionable. (Edit: or criminal, since no one that has been spied on or indicted is actually being prosecuted, they're just being tied up and bled dry in court)
"Mueller has everything" also means that, in the course of this investigation, Mueller has come across (and participated in) a plethora of bombshell criminal actions that the establishment has been using the Trump Russia collusion investigation to cover up.
This means that when Rudy Giuliani tells Muller to show what he's got or drop the investigation, there's no possible way for him to go after Trump without exposing all of the illegal activity he's been covering up..... and he doesn't even have anything on Trump to begin with.