One9,
Do you know your basic major scales?

You will need to know your scales in order to understand the circle.
Once you know your scales, then you can start on any key on the board, take a perfect fifth of that note, and keep doing that in succession until you get back to the original note. When you get to each perfect fifth, you play a chord based on that note. You can use this method to practice your playing.
You can do the same thing with fourths. The fourths go one way around the circle, and the fifths go in the opposite direction.
HTH,
God bless,
eggs