When we remove a cataract at surgery we leave a small membrane (posterior capsule) behind in the eye that supports the implant.
At the end of cataract surgery this membrane is usually perfectly clear and transparent but in some people as the months or years go by this membrane can thicken up(opacify) and start to obscure the vision again.
The YAG laser is able to make an opening in this thickened capsule or membrane. This enables you to see through the opening and improve your vision again assuming the rest of the eye is healthy.