Systemic means the chemical can either move through the plant from the roots to the foliage or within the plant when applied to the leaf. Systemic weedkillers are good at killing underground roots of perennial weeds. Systemic insecticides can kill insects hidden from contact with the spray.
Contact means that the chemical does not move but stays where it is sprayed. Contact products need to be carefully sprayed to get good cover of the target weed or insect infested plant.