The Circle Equation

Ah, yes. High school flashbacks.

The Circle Equation relates x and y in the following general equation:The point on the graph formed by taking the coordinates a and b, that is, (a, b) will be the centre of this circle.

Notice that we have this on the right side: As you probably have understood, talking about circles, this R represents the radius of this circle.
We have a point that represents the centre of this circle and we can also get the radius from the equation.

This lays the foundation you'll need to understand the next concept:
The Unit Circle