How to Choose value of Output Frequency Carrier for PWM
The article is about a timer with an 8 bit counter; if you set the counter to count to its maximum of 255, the PWM frequency you get is:
\$f_{\small PWM}  = {\Large{f_{clock}} \over {\large prescaler \times 255}}\$
This is because the counter has to count to 255 for every period of the PWM, so the PWM frequency is \$ 1 \over 255\$ of the clock frequency (assuming the prescaler is set to 1).
If you want phase-correct aka centre-aligned PWM, the counter will count up to 255 and then down again to 0 for each PWM period, so it will take 2 x 255 = 510 clock ticks for each PWM period, which explains the 510 in the formula you quoted.