Released on the 2nd of February, the Nunes memo alleges serious impropriety on the part of the FBI, claiming that they requested and received FISA warrants illegally by misinforming the judge responsible for overseeing this particular investigation (on Carter Page), using information acquired from the Steele Dossier, but not informing the judge of its political origin and leaning. Supposedly this renders the investigation itself invalid, and from there somehow renders the separate Mueller investigation defunct or corrupt or some-such.
So, firstly the investigation on Carter Page by the FBI was not precipitated by the Steele Dossier. The memo itself states that it was in fact another aides bragging session with and Australian diplomat, regarding his and the Trump campaigns Russian contacts, that first begun the investigation and inquiry into Carter Page. Additionally, Nunes himself confirmed a few days after the release that on issuing their request for FISA warrants for Page, the FBI did note that the dossier where some, not all, of the information presented to the court to provide reasoning for a warrant did in fact come from a political source. Apparently that was not enough to invalidate it. Finally no, the FBI investigation is neither defunct nor corrupt, and Mueller was not involved in this matter in any way until months later when his own investigation began. I don’t think the memo even mentions him.
So basically the Nunes memo is is kind of irrelevant. While it alleges significant things, not only does it refute them itself, the author has denied certain points, the FBI and the Justice department have both named the memo false and misleading, and most people (other than Republicans) agree that while the FBI is certainly not squeaky clean, they are professionals who would never do the things the memo alleges. It did however suck a lot of the air out of the room, and made people forget that Trump threatened to fire Mueller not long ago.