Between two capacitor plates there is a homogeneous electric field $$E$$. Now, if an electric dipole (e.g., a molecule with a positive and negative charge) is placed in the homogeneous E-field with the distance $$r$$ of the charges, then the negative charge experiences a force $$F_1$$ toward the positive capacitor plate (against the E-field) and the positive charge experiences a force $$F_2$$ toward the negative capacitor plate (along the E-field). So the dipole rotates in the field until it is exactly parallel to the electric field lines, because its energy then becomes minimal.