"Runny batter" refers to a mixture that is thin and liquid-like in consistency, often used in baking or cooking. It flows easily and doesn't hold a rigid shape like thicker batters. Full definition
Bake brownies until firm to the touch and a tester inserted into the center comes out with moist crumbs (not runny batter) attached, 15 — 18 minutes. (epicurious.com)
It is a fairly runny batter, and mine was done in 15 minutes. (backforseconds.com)