If you type in google search “sin(x)” it will show you a graph of sin(x). Sin, cos and tan are periodical functions that will go on and on with the same frequency. These functions use degree or radian input, depending on the function type that is used.
To get a better understanding about the radian part you can look at this:
A circle with a radius of 1 exists of 2pi radians (in other words 360 degree = 2pi). Both methods (degrees and radians) will give the same result when used in your calculations, only the function that you use will require 1 method all the time.
To understand the output of the sin and cos can be visualised with the following image:
When a vector with length 1 is rotating counter clockwise around the origin (0,0) the function cos(x), where x is the angle or radians, will give the X value of the angle/radian. The sin(x) function, where x is also the same angle or radian for the cos(x) function, will give the Y value of the same angle/radian. When the angle starts at 0 degree or 0 radians the vector will be pointing to the right. At this point the cosine function will output 1, because cos(0) = 1, and the sin function will output 0, because sin(0)=0. This can be checked in the previous picture (the blue vector will be at his maximum length and the green vector will not be visible at all so the length will be 0).
For a better understanding take a look at this gif animation about the sin, cos and tan function:
I hope this will give a better understanding about the sin and cos functions. If you want to get a closer look at the functions check the wiki