We can model a real world situation by applying ratios. If you want to know the height of a building and can measure its shadow on the ground and the angle of elevation of the top, then apply the tangent formula (tan of angle a = side opposite angle a/side adjacent to angle a).
There are five things to know in a right-angled triangle -- the length of the the three sides, and the other two angles. If you know any two then the sine, cosine and tangent ratios will give you others.
Given angle a:
Tangent of a = opposite side/adjacent side.
sine of a = opposite side/hypotenuse.
cosine of a = adjacent side/hypotenuse.
Break down the model of the real world problem into triangles. If the triangle does not contain a right angle, divide it in two by dropping a perpendicular onto a base line and treating it as two right angle triangles.