Every action has an equal but opposite reaction.
So, if I push on a door with a force of 5 N, the door will push back on me with a force of 5 N.
Newton's third law states that all forces occur in pairs such that if one object exerts a force on another object, then the second object exerts an equal and opposite force on the first.
Summed up, this means that "For every reaction, there is an equal and opposite reaction."
I hope this helped!