A flat line on a speed-time graph signifies that the object is moving at a constant speed.

In a speed-time graph, the vertical axis represents speed, while the horizontal axis represents time. When a flat, horizontal line is observed, it indicates that the speed of the object remains unchanged over time. This occurs because the y-value (speed) stays consistent as time progresses along the x-axis. In essence, the object is neither accelerating nor decelerating; it is maintaining a steady speed.

For instance, if a car is traveling at a constant speed of 60 km/h, the speed-time graph would depict a horizontal line at the $60 \text{ km/h}$ mark. This indicates that the car is neither speeding up nor slowing down, but rather moving at a uniform speed.

Grasping this concept is vital for interpreting motion in both physics and real-life situations. It aids in analyzing scenarios where maintaining a constant speed is crucial, such as with cruise control in vehicles or conveyor belts in manufacturing. Furthermore, recognizing a flat line on a speed-time graph can assist in solving problems related to distance traveled, as the area beneath the line (which forms a rectangle in this case) represents the distance covered over a specified time interval.

Thus, whenever you encounter a flat line on a speed-time graph, remember that it signifies constant speed. This understanding is straightforward yet essential for comprehending motion.