Displacement differs from distance because it includes direction. If you walk 5 m north and then 5 m south, your total distance is 10 m but your displacement is 0 m (you are back where you started).
Displacement is a vector quantity. Distance is a scalar. Speed uses distance; velocity uses displacement.
On a displacement-time graph, the gradient gives the velocity. A negative gradient means the object is moving back toward the starting point.
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