The slope of the distance-time graph indicates speed. It is because the slope of a distance-time graph determines the speed of that body, so the steeper the slope greater will be the speed of the body.
What is a distance-time graph?
A distance-time graph as the name suggests indicates the distance travelled by an object in a given time. In this graph, the distance is considered on Y-axis and Time on X-axis. If the graph is a straight line then the motion of the object is uniform. The slope of the straight-line graph is the same irrespective of the interval which is chosen. This implies that the speed of an object under uniform motion remains constant.