I think I know what you mean. When you have any graph or equation relating variables, in this case, y=-½x, this is a continuous line with no beginning and no end. But when you are told to build a table of values you want to know where to start, right?
To draw a linear graph (a straight line), you only need two points to join up and then you draw the graph through these points. The two easiest points to draw are the intercepts—where the line cuts the axes. To find out that you put x=0 and find y then put y=0 and find x. In this case x=0 when y=0, so the line goes through the origin (0,0). Now you can need any other point, so to get an accurately drawn line pick a value of x a few units away from the origin. Let’s choose x=4. Then y=-4/2=-2. You now have two points you can join together: (0,0) and (4,-2). Draw a straight line that passes through these points and that gives you the graph. You can set up a table with other points (if you choose any even values for x it’s easier in this case) and you will find that they all lie on the line. You can’t draw a line of infinite length, of course, but a limited graph shows that you understand that the line continues beyond the bit that you draw. In this case you will find that the line leans backwards. Your teacher wants you to show the shape and direction of the line, if a graph is required.