Imagine slicing up your polygon like a pizza.
The central angle is the angle of the pizza slice, equal to 360º divided by the number of slices.
The apothem is the distance from the center of the "pizza" to the middle of the crust on one slice. Imagine putting a ruler down the middle of your slice of pizza.
Now imagine measuring the length of one of the sides of your slice of pizza (but not the crust side) - this is the radius.
Since real pizzas are circles, the apothem and the radius are equal. But if your pizza is a regular polygon, the apothem is a little shorter than the radius.