When you walk your foot pushes on the floor and the floor pushes back on you – in the opposite direction, resulting in your moving in the direction you are walking.
Newton’s 3rd law of motion states that for every action there is an equal and opposite reaction. That means that for every interaction between objects there are always 2 forces acting. The forces must be equal in magnitude. That is they must both be equally “strong.” They also must be opposite – working in opposing directions.
When a bird flies, its wings push down on the air. The air pushes up on the wings. When you drive a car, the wheels put backwards on the road That means the road pushes forward on the wheels.
It’s a little difficult to understand that this happens anytime you have an interaction between objects. When you sit down on a chair, the chair pushes up on you. And, of course, when you hit your brother, he’s going to hit you back.
This video will provide additional clarification.