When working with trochoids what does θ stand for?

Question

These are the formulas with which you can draw trochoids.

$x = aθ - b sin(θ)$

$y = a - b cos(θ)$

I'm trying to make trochoids but I got hung up on this symbol $θ$, what is it and how do I use it, I understand what $a$ and $b$ stand for in there formulas but I'm having a hard time finding out what I should fill in for $θ$.

Answer 1

As $x$ is the variable for function $f(x)$, $θ$ is the variable of your parametric function.