Explain translational motion by the given illustration.

(N/A) rectangular block is shown sliding down an inclined plane $AB$ at an angle $\theta$ without any sidewise movement.
In this motion,the block moves down the plane such that all particles of the body (e.g.,$P_1$ and $P_2$) move together. This means that at any instant of time,every particle of the rigid body has the same velocity vector.
Such motion of a rigid body is defined as pure translational motion.
Therefore,in pure translational motion,at any instant of time,all particles of the body possess the same velocity.