Trigonometry is the study of triangles. Three trigonometric functions that are specific to right-angled triangles are sine, cosine and tangent, often shortened to sin, cos and tan.
Firstly it is important to understand the terms used to describe the three sides of a right triangle:
Hypotenuse – The side opposite the right angle, always the longest side
Opposite – The side opposite the angle you’re working with
Adjacent – The side next to the angle you’re working with
Sin = Opposite / Hypotenuse
Cos = Adjacent / Hypotenuse
Tan = Opposite / Adjacent
The acronym SOHCAHTOA is helpful to remember which sides go which each function.
These functions can be used to relate the angles in the triangle to the side lengths.