The slope of the line is the velocity, or rate of change, of y with respect to x. In other terms, the slope tells you how much y will change for every change in x by one unit. For example if the slope, which is velocity, is 2. y will increase by two units every time x increases by one unit. Your graph is not pictured nor explained, so I cannot give you the specific context beyond the principle that the slope of a line is a constant velocity or rate of change.