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That's the speed.
The steepness of the line on a distance-time graph represents the radial speed of the object. That is, the speed with which the object is moving towards or away from the origin. The steepness takes absolutely no account of the transverse speed, so you can be going around the origin in a circle at a great speed but, since your distance remains the same, the D-T graph will be flat: implying speed = 0.
Circle Graph
circle graph
assuming you're speaking of a horizontal line on a graph: It is because the line moves neither up or down. slope is the steepness of a line and a horizontal line isn't steep at all, it has no steepness.