When two bodies interact, they exert a force on each other-
If object A exerts a force on object B, then object B exerts the exact opposite force on object A.
This means, if you push something, e.g. a shopping trolley, the trolley will push back against you, just as hard.
As soon as you stop pushing, so does the trolley.
The force of you pushing the trolley is called the action force.
The force of the trolley pushing back against you is called the reaction force.
If the forces are always equal, how does anything ever go anywhere? Think of a pair of ice skaters: