Trigonometry is based on the study of right triangles, which must contain a right angle. Those who study trigonometry use the theta symbol as a point of reference to other angles within a triangle.
There are six trigonometric functions: sine, cosine, tangent, cosecant, secant and cotangent. When someone attempts to apply one of these functions to an angle, the theta symbol is used to denote the equation. The symbol is drawn next to the angle in question, and the person working on the trigonometry problem knows which angle to solve for. Alternatively, the phi symbol is sometimes used instead of theta.