This means your velocity is decreasing with time, or in other words, the object is slowing down.
It means acceleration.
Slowing down or decelerating
If you are analyzing speed with respect to time, then you are decelerating.
If you want the graph to show the acceleration of the ball against time, then the graph is a horizontal line. If you want the graph to show the velocity of the ball against time, then the graph is a straight line sloping downward. If you want the graph to show the height of the ball against time, then the graph is a parabola that opens downward.
It means that either the distance is measured from the starting-line and the object is moving forward, or else the distance is measured from the finish-line and the object is moving backwards, because the distance is growing as time goes on. If the upward sloping diagonal line is straight, it means the speed is constant. (not velocity)
That means the speed (the slope of the position-time graph) is decreasing.
The downward tend on a graph is called "decay".
On a graph of acceleration vs. time, during deceleration the line is below zero. On a graph of speed vs. time, during deceleration the line has a negative slope (sloping downward from left to right).
An upward sloping straight line indicates that the object being studied is moving away from the origin and that the component of its velocity in the radial direction is a constant. A downward sloping line indicates it is moving towards the origin. However, neither line says anything about the transverse component of its motion.
Acceleration due to gravity (free fall) is a downward (negative) motion. On a position-time graph, the slope representing free fall will be a downward curve. On a velocity-time graph, the slope representing free fall will be a straight line with a negative (downward) slope.