What do double solid yellow center lines generally mean?

Correct answer: Crossing them to pass is generally prohibited.

Why

Double solid yellow center lines generally mark opposing traffic where crossing to pass is prohibited. The handbook also permits a safe left turn across them. Current Massachusetts law has an additional conditional passing rule: if a vulnerable road user or another vehicle cannot be overtaken at a safe distance within the lane, the passing driver uses all or part of an adjacent lane, crossing the centerline if necessary, only when safe and without exceeding the speed limit. The marking is not permission to disregard oncoming traffic or other applicable restrictions.

Verified against Massachusetts Driver’s Manual · Yellow lane lines · p. 98