The document discusses the sine law, which states that in any triangle, the ratio of the length of a side to the sine of its opposite angle is a constant value. It provides examples of how to use the sine law to solve for a missing side or angle in a triangle when two sides and their included angle or two angles and the included side are known. The document also discusses the ambiguity that can arise when using the sine law to solve for an angle, as there may be two possible angle measures that satisfy the ratios.