This article by Steve Eder explains the situation. A notice sent out at the class-certification stage in 2015 gave class members the right to opt out — as is required in all Rule 23(b)(3) class actions (such as the Trump University class action).
But now a class member named Sherri Simpson is seeking to opt out at the settlement stage. Simpson is apparently asking the judge, Gonazalo Curiel, to invoke Federal Rule of Civil Procedure 23(e)(4), which provides:
The following procedures apply to a proposed settlement, voluntary dismissal, or compromise: . . . If the class action was previously certified under Rule 23(b)(3), the court may refuse to approve a settlement unless it affords a new opportunity to request exclusion to individual class members who had an earlier opportunity to request exclusion but did not do so. (emphasis added)