Depends on how you create the frequency. My code was software based, so the resolution was limited to what micros() offers, which will be 4 to 8 microseconds.
Using PWM, which uses hardware in a kind of set & forget configuration so no software involvement, you can obtain other frequencies.
The 0-255 refers to the width of pulses with a set period, but the frequency itself is fixed.
For example, with analogWrite, the frequency is ~488 Hz, about 2mS period, so the high pulses can be 1/255 of 2mS wide, up to 254/544 of 2mS wide.
Google "PWM secrets" for articles on setting up prescalers & the timer/counters for PWM frequencies.