Sine, cosine and tangent each describe a fixed ratio between two of a right triangle's three sides, taken relative to one of its acute angles: sine is the ratio of the opposite side to the hypotenuse, cosine is the ratio of the adjacent side to the hypotenuse, and tangent is the ratio of the opposite side to the adjacent side. What makes these three ratios genuinely useful is that each one stays exactly the same for a given angle no matter how large or small the actual triangle is, which means knowing just one angle and one side length is enough to calculate every other side.
Similar triangles are the reason the ratio stays fixed regardless of triangle size
Any two right triangles that share the same acute angle are similar triangles, meaning they have exactly the same shape even if one is much larger than the other, and similar triangles always keep the same ratios between corresponding sides regardless of overall size. That's exactly why sine, cosine and tangent, defined as ratios rather than as raw lengths, can be tabulated once for each possible angle and then applied to any right triangle containing that angle, a tiny triangle drawn on paper or an enormous one used to measure a mountain's height, with no adjustment needed for scale at all.
Knowing an angle and one side is enough to unlock every remaining measurement
Given one acute angle and the length of any one side, sine, cosine and tangent let you calculate the other two sides directly: rearranging whichever ratio actually involves the known side and the side you want gives a straightforward equation to solve. This is exactly the practical power trigonometry adds on top of plain geometry: rather than needing to physically measure every side of a triangle, a single measured angle and a single measured side, often much easier to obtain in practice, unlock every other measurement the triangle actually has.
What we're still unsure about
That sine, cosine and tangent remain fixed for a given angle regardless of triangle size, following directly from the properties of similar triangles, is well established, rigorously proven geometry going back over two thousand years. What's more genuinely a matter of pedagogical sequencing is exactly how to introduce these ratios so students grasp why they're constant, not just that they are, since simply memorising the three ratio definitions without understanding the underlying similar-triangles argument tends to produce fragile, easily forgotten knowledge, and maths educators continue to develop different teaching approaches aimed at making that underlying reasoning genuinely stick rather than treating the ratios as arbitrary rules to be looked up.
This sits inside Sine, Cosine & Tangent, one of seven topics in Trigonometry, one of seven domains in Mathematics, one of seventeen subjects the app can quiz you on.