The most common cause of this symptom is a cloudiness in the capsule that was left behind to support the Intraocular Lens(IOL). The treatment is very straightforward, but ideally should be delayed for 60-90 days post cataract surgery in order to allow the new lens to settle into the proper position.
The film behind the implant is referred to as posterior capsular haze. The generally safest time to have it layered (Yag) is after 90 days, once the implant (IOL or intraocular lens) has had a chance to scar into place. Doing the laser before this time could cause dislocation of the IOL or vitreous to wrap around from the back of the eye in front of the implant.