Newton's third law says that when object A exerts a force on object B, object B exerts an equal and opposite force on object A. The two forces are the same type of force, the same size and in opposite directions. For example, if skater P pushes skater Q with 50 N to the right, Q pushes P with 50 N to the left.
The two forces in a pair always act on different objects, so they never cancel each other out. A book on a table is an example: the table pushes up on the book, and the book pushes down on the table. The book's weight is the pull of Earth on the book, and its third law pair is the book pulling up on Earth. The book stays in equilibrium because the forces on the book alone are balanced.
In a collision the two objects exert equal and opposite forces on each other for the same time. So their changes in momentum are equal and opposite. This means the total momentum of the objects is conserved: it is the same before and after the collision, as long as no external forces act.
