How is the concept of speed being relative explained in physics?

In physics, the concept of speed being relative is explained by the theory of relativity, proposed by Albert Einstein. According to this theory, the speed of an object is not absolute, but depends on the observer's frame of reference. This means that an object's speed can appear different to different observers depending on their relative motion. The theory of relativity helps to explain how time, space, and motion are interconnected in the universe.