These can be a bit tricky. The teeny 1 there tells us this grid is 1x1 cells. The open circles at the end of the segments indicate the endpoint is not part of the function. The filled circles indicate that endpoint is part of the function.
The origin is where the x and y axes meet. f(-3) means we start at the origin and count three units to the left, left being the negative direction. Then we eyeball the figure to see where the vertical line x=-3 intersects f. It's at the black dot in the lower left quadrant, which is four down from the y axis, so the point (-3, -4)
