This node enables you to set the velocity of particles. Velocity is a vector; it incorporates the speed (which is the length of the vector) as well as the direction (alignment of the vector). So, using this node, you can set both the speed of particles as well as the direction in which they are travelling.
This node takes priority over other nodes that affect velocity. In other words, no other node will be able to change the velocity of the particles while the particles are connected to a PVelocity node.