In geometry, coplanar refers to objects lying in the same plane. Coplanar would mean two substituents lying in the same plane. That is, both are wedged or dashed or in the plane of the paper (sticks). For example, if you want to do E2, you would need the leaving group (ex: tosylate) and the nucleophile (ex: hydroxide) to be in the same plane and anti at the same time due to transition state energy and steric hindrance effects.