Ethernet is a LAN(local area network) that connects devices via a shared medium. The size of the Ethernet is confined to less than 2.5 km. The reason for the size limit is because Ethernet needs to run collision detection and for the worst case scenario, two devices at two opposite ends send frames at the same time and if it takes d times to go from one end to the other, then in the worst case scenario, it will take 2d times to detect the collision.
So the sender has to keep transmitting after 2d time so it can jam the frame.