What does no mass effect mean in regards to a cranial ct scan?

It means: They are not seeing any shift in midline structures or distortion of the ventricular system.

Mass effect: Can be seen as an asymmetry of normal brain structures. When problems occur in the brain (e.g., tumor, hemorrhage, stroke) the problem itself and/or any associated swelling can compress parts of the brain and distort the normal anatomy, especially the fluid-filled spaces like the ventricles and sulci. Thus, "no mass effect" indicates that the normal symmetry of the brain was seen on the ct scan.