In a right triangle, there are three sides, these sides are based upon which angle you reference yourself from. Two sides are variable, one is not.
No matter where you reference yourself the hypoteneuse(deonted h) is always the same, but the opposite(denoted o) and the adjacent(denoted a) sides change depending from which angle you are working off of.
These three functions you speak of are geometrically defined as the RATIO between the sides of a right triangle. So for example
This means that the cosine of the angle you are at is the decimal representation of the ratio between the hypoteneuse and the side opposite of your angle..
Or in other words a decimal represenation of the ratio between the two sides
This is the same thing as the others but for different sides