The slope (gradient) of a speed v time graph is
If the graph is a straight line, then the acceleration is
constant. if the slope is 0 (ie the line is horizontal) then the
acceleration is 0, and the object is moving with a constant speed
(which can be 0, ie the object is stationary).
If the graph is "slanted" then depending upon which way the line
is "slanted" the object is accelerating or decelerating (the
steepness of the slope gives the rate of this).
So the object is moving with constant acceleration and either
speeding up or slowing down - which, and by how much, depends upon
the gradient of the graph.