check the picture below.
recall that a reference angle, is the angle made with the x-axis, regardless of the Quadrant the terminal point is at.
what's its tangent? well, you can check your Unit Circle, if you don't have one, you'd want to get one about now, you can search online for one, quite a few to get there, anyhow, at that point, the "x" and the "y" are the same value, and thus the tangent is 1.
