What is a reference particle and a reference circle?

(N/A) reference circle is a circle of radius $A$ (where $A$ is the amplitude of the simple harmonic motion) centered at the origin of a coordinate system.
$A$ reference particle is a hypothetical particle that moves with a constant angular velocity $\omega$ along the circumference of the reference circle.
The projection of the position of this reference particle onto any diameter of the reference circle executes simple harmonic motion $(SHM)$. The displacement of the $SHM$ is given by $x(t) = A \cos(\omega t + \phi)$,where $A$ is the radius of the reference circle and $\omega t + \phi$ is the angular position of the reference particle at time $t$.