How is velocity relative?

Velocity is a relative property, meaning it is defined only in relation to other objects. The theoretical physicist Carlo Rovelli explains this concept using the example of a mother telling her child not to move while on a train. In this context, "not moving" means having zero velocity relative to the train, not the Earth or the sun. Essentially, velocity is not an inherent characteristic of an object; it always requires a reference point. This idea extends to quantum mechanics, where all properties are relative between objects 1.