The announcement doesn't specify what constitutes the eligible level of need, but indicates that 45-50 students (out of about 600 in the law school) will get the scholarships to cover all their tuition.  So that's a significant investment! 

UPDATE:  My colleague Ryan Doerfler calls my attention to this informative piece, with further details:

The plan, according to The Wall Street Journal, includes scholarships worth roughly $72,000 to cover tuition, fees and health insurance for students from families with incomes below the federal poverty line, which is currently $27,750 for a family of four….

Yale said about 8 percent to 10 percent of its currently enrolled law students are eligible for the award, known as the Hurst Horizon Scholarship, according to the newspaper.