waves are related to circles: if you have a line and anchor it at one end, when you rotate it the other end of the line, it draws a circle, but if the paper you're drawing it on moves to one side at a constant speed, you'll get a wave. Alternatively, if you plot where the other end of the line is as time passes (for example, every second or every minute), you'll get a wave. you can do this in reverse too.

it's helpful to convert to circles. from a regular wave, at 0 you don't know if the wave will go up or down without further information. 0 on a circle will correspond to one of two spots, either the very top or the very bottom, and if you know which direction the circle is rotating, you can tell what the related wave will do next.

at least that's my understanding