In trigonometry, the unit circle is a circle with radius 1, centered around the origin of a coordinate plane. Seems simple but, it helps in the understanding of trigonometric functions, the basis of high school trigonometry.
Trigonometry is seriously studied in the 11th or 12th grade and can throw students for a loop in the same way that Geometry does. The unit circle is a fantastic reference for them. The interactive unit circle built by Touch Mathematics does much, much more than any static image could do. For students, connecting concepts in math is hugely important in order to gain a mastery over the content. The interactive unit circle helps students to connect many of the concepts at the root of trigonometry: basic trigonometric functions, periodicity of trig functions, angle relationships between trig functions...the list goes on and on.
In a high school math classroom instead of the traditional, static, image of a unit circle that is handed out at the beginning of the year or pointed to in the appendix of some textbook imagine being able to pull this up each time a new concept is introduced that involves trig functions.