Linear Momentum of a body of mass m is defined as the product of Mass m and its Velocity v. Its direction is same as that of velocity.
When there is no external force acting on an isolated system of bodies, then the total linear momentum of the system remains the same.
If m1, m2, m3 have velocities u1, u2 and u3 at a time t = t1 and v1, v2, and v3 respectively at time t = t2 then
m1 u1 + m2 u2 + m3 u3 = m1 v1 + m2 v2 + m3 v3
IF two foot balls are thrown at each other in a field, they collide in air, and they rebound back with speeds according to the law. If they are thrown with velocities u and v towards each other and they have a perfectly elastic collision, then the first one rebounds back with spped v and second one rebounds back with speed u. their speeds are interchanged.
If you walk on a stationary boat, you move forwards on the boat, the boat moves backwards on the water.