A dimmer is either a "phase-cut" device, either (commonly) forward-phase/leading-edge or (less commonly) reverse-phase/trailing-edge. At all times, it's either on or off, and it cycles between on and off once per half wave, so it produces 120 pulses per second. A good-quality light fixture will smooth out that waveform and produce approximately constant output.

The worst choice is a no-neutral-required forward-phase dimmer. Neutral-required forward-phase dimmers are usually better. Reverse-phase dimmers can be excellent for LEDs (but disastrous for magnetic transformers) and always require neutrals. Some dimmers can operate in both modes.