"Trigonometry is a branch of mathematics that studies relationships between side lengths and angles of triangles." - Wikipedia
The first things you will probably learn are sines, cosines, and tangents, which basically show how one angle relates to some side lengths using the functions sin(x), cos(x), and tan(x).
Here is an example(you will not need to understand, just a sneak peek of usefulness of trigonometry):
Let us denote a right triangle with hypotenuse BC, and A as the vertex with the $90^\circ$ angle, B as the angle above the $90^\circ$ angle, and C as the rightmost angle. If $sin(B)=12$, and the hypotenuse is of length $2$, then what is side length $AC$?
The answer is 24, because $sin(B)=\frac{AC}{BC}$
You do not need to understand this, this is just a sneak peek of the usefulness of trigonometry.
Hope this helps! For more definition, here is the link to the wikipedia article: https://en.wikipedia.org/wiki/Trigonometry