Frequency and duty cycle
Frequency is how many PWM cycles occur per second. Duty cycle is the fraction of each cycle that the signal remains high. A 25 percent duty cycle is high for one quarter of every cycle.
For an LED, rapid switching is perceived as brightness. For a DC motor used with a proper driver, average delivered power influences speed.
PWM is still digital
The pin is not producing every voltage between 0 V and 3.3 V. It switches between low and high. The load, your eyes, or an added filter averages those pulses.
Choosing a frequency
The useful frequency depends on the load. LEDs should switch fast enough to avoid visible flicker. Standard hobby servos usually expect a roughly 50 Hz control frame, but their position depends on pulse width rather than ordinary brightness-style duty control.
When you need extra hardware
Never drive a motor directly from a GPIO pin. Use a transistor, MOSFET, or motor driver plus appropriate power protection.