The opinion’s introduction summarizes the holding as follows:

We hold that the police unions’ motions to intervene are untimely and do not assert an interest that the law seeks to protect. The unions knew, or should have known, of their alleged interests in these controversial and public cases well before they filed their motions in September 2013. For years now, “stop‐and‐frisk” has been the subject of extensive public filings and intense media scrutiny. Whatever the merit of the unions’ claim that Judge Scheindlin’s rulings were incorrectly premised “upon statistical evidence purporting to place 4.4 million stops at issue,” allowing the unions to revive a now‐settled dispute by intervening at this late juncture would substantially prejudice the existing parties and unduly encroach upon the City’s inherent discretion to settle a dispute against it. In other words, granting the unions’ motions in the wake of the November 2013 mayoral election would essentially condone a collateral attack on the democratic process and could erode the legitimacy of decisions made by the democratically‐elected representatives of the people. Furthermore, the police unions’ interests in their members’ reputations and collective bargaining rights are, as a matter of law, too remote from “the subject of the action” to warrant intervention as a “party.” We stress that our holding is limited to the particular and highly unusual circumstances presented here and should in no way be construed to encourage premature intervention in cases of public concern where government defendants have discretion to settle.

Judge Torres’ prior ruling was covered here, and other posts on stop-and-frisk are here.