Sin, Cos & Tan
The ratios between sides of triangles are called sine, cosine and tangent, normally shortened to sin, cos and tan.
You need to know these ratios! The word soh-cah-toa (spoken in a Chinese manner) can help you learn them, or try the mnemonic
"Step On His Corns And Hack The Other Ankle".
Why's this important? If an angle is known, so too is the ratio between its sides. This means that if you know how far a mountain is from you, and you can measure the angle of elevation to its peak, you can work out how tall it is!