The reference angle is the positive acute angle that can represent an angle of any measure.

The reference angle $$ \text{ must be } < 90^{\circ} $$.

In radian measure, the reference angle $$\text{ must be } < \frac{\pi}{2} $$.

Basically, any angle on the x-y plane has a reference angle, which is always between 0 and 90 degrees. The reference angle is always the **smallest** angle that you can make from the terminal side of an angle (ie where the angle ends) with the **x-axis**. A reference angle always uses the x-axis as its frame of reference.