A vocoder is an effect that analyzes the spectral shape of one signal, the modulator, and imposes it on another, the carrier. Typically, a voice is the modulator and a harmonically rich synth is the carrier, which gives the classic talking synth sound.
It does this by splitting both signals into frequency bands, tracking the level of each band in the modulator with envelope followers, and using those levels to control the same bands of the carrier.