this means that whenever one object pushes on a second objact the same amount for example if you try and push a big truck while wearile a pair of roller blades then you will move backwards when you push the truck your body applies a force to the truck and the truck in tum applies a force to your body since the truck is a lot heavier than you it doesn't move very far but you move backwards a long way because you accelerate move and that's what the second law says!