The range involves the y-axis, whereas the domain involves the x-axis. The range is the area between the highest point and lowest point. So for this question, the highest point is 1 and there is no lowest point because the line doesn't end. If there's an open circle on the point the sign changes to <, but a closed circle is ≤. That's how we get f(x) ≤ 1.
I know it's confusing, but I hope this helped a little bit.
