This could be a situation in which items are both bought and sold. In calculating the inventory, positive numbers represent items bought, negative numbers represent items sold and 0 would be no items in the inventory.
Another type of situation would be the displacement of an object from the original position. Where 0 would represent the original/starting position, positive values would represent distance moved forward, negative values represent distance moved backwards.
The line is positive because it is going upwards. The y-intercept is positive 3. It can be found by seeing where the line intercepts the y-axis. The slope of the line is 2 and it can be found by using "rise over run."
(I saw the image on another question you had asked. Have a nice day!)
