In order to help you understand the significance of the helical angle in endodontic files I have to start at the beginning and review two significant portions of high school science with you. I'll feed it to you in small bites to make it more digestible.
Trigonometric functions are ratios between two sides of a right triangle.
A right triangle is on in which one of the sides is a 90 degree or right angle. As you will recall, the total of all the angles of a triangle must equal 180 degrees, therefore if one of them is 90 the sum of the other two must equal 90. If you think about it, if there are three sides of a triangle then there are six possible ratios between the lengths of the sides. In trig these sides of the triangle are not named A,B and C but Adjacent, Opposite and Hypotenuse to reflect their location relative to the angle we refer to. You can visualize the triangle as sitting on one of its non-hypotenuse sides with the right angle to your left, and the angle we are concerned with at the lower right.