Velocity is a vector quantity which has both magnitude and direction whereas speed is a scalar quantity which has only the magnitude. To put this in simple terms, velocity is the speed at which an object moves in a particular direction, such as the speed of a bike traveling north on a major freeway, or the speed a rocket moves as it launches into space. The displacement of an object in a certain amount of time means velocity and that velocity has units of distance divided by time, such as miles per hour or kilometers per hour or meters per second. The dimension of velocity is given by L T-1. The change in position divided by the time of travel is said to be average velocity.
Velocity measures the motion of an object that starts moving from one place and starts heading towards another place. When we put it in the other way, you use measures of velocity to decide how quickly you or any object in motion will arrive at a destination from a given from the starting location.
Relative motion is just a form of saying that sometimes various people will say distinctive things about the motion of the same object.
Let us say I am standing on the back of a pickup truck such that they are motionless, and I am throwing oranges forwards. I know that I can throw an orange at exactly 12m/s every time.