The phrase "to thicken the batter" means to make the batter thicker or more viscous by adding ingredients or changing its consistency, usually to improve the texture or make it easier to work with in cooking or baking. Full definition
Nutritional yeast helps thicken the batter, giving this recipe all the comfort food qualities of the classic dish. (dailyburn.com)
I added an extra cup of coconut flour which thickened the batter a bit and then poured it into a muffin - top pan to bake the cookies. (wholenewmom.com)
(That step will help to thicken the batter somewhat.) (gfjules.com)