The object is slowing down between 1 and 3 seconds.
In a velocity vs time graph, the slope of the line represents the acceleration. A positive slope indicates positive acceleration (speeding up), a negative slope indicates negative acceleration (slowing down), and a horizontal line indicates constant velocity.
The slope of the line in the graph you sent is negative between 1 and 3 seconds. This means the object's velocity is decreasing during that time interval, so it is slowing down.
