If we start with a note C and simply add the interval of a perfect fifth to it iteratively, we end up generating a set of frequencies that are very close to the current major scale: C - G - D - A - E - B - F-sharp - C-sharp - G-sharp - D-sharp - A-sharp - E-sharp (or F), and then back to C. This is known as the circle of fifths because after going through the cycle, we end up back at the note we started on.
