What do weak motor skills mean?

The terms you refer to means that the motor (muscle, in lay terms) milestones are delayed. It further needs to be determined by the doctor whether other milestones involving his fine motor skills, social and language milestones are age appropriate or not. Then your doctor needs to find the cause: is this related to any disorder of the brain, nerves or muscle. Having done that, appropriate therapy needs to be instituted. I suggest you do not ignore this observation by your doctor. If available, a Child Neurologist should be consulted at the earliest.