A quantizer is a module that maps a continuous or analog signal to a stepped signal, like the notes on a keyboard. Essentially, it takes a continuous range of values and divides it into a finite number of discrete levels or "bins." Each bin represents a specific range of values, and any input within that range will be mapped to the same output value. This process is called quantization, and the output of a quantizer is a quantized signal.

The quantizer divides the input signal into predefined intervals or "steps," and then maps the input signal to the closest step in the set. For example, a pitch quantizer may divide the pitch range into semitones and map any incoming pitch to the nearest semitone. This ensures that the output pitch will always be a particular, in-tune note rather than a slightly out-of-tune pitch that may occur with an unquantized input signal.

Some Eurorack modules also allow for different quantization modes, such as chromatic, where all notes are quantized to the nearest semitone, or a specific scale, where only certain notes within the scale are allowed. This allows for more creative possibilities in terms of melody and harmony.

In summary, a quantizer in a musical synthesizer context is a device that converts a continuously variable input signal into a discrete, fixed value, ensuring that the output is always a precise, in-tune note. This can be useful for creating melodies and harmonies more predictably and consistently.