Chorus effects thicken your signal by copying it multiple times, coloring the copied signals, and playing them back slightly delayed.
Chorus is a type of modulation effect that functions very similarly to flanger pedals. As its name suggests, a chorus pedal's purpose is to thicken and color your signal enough to sound as if there are multiple guitars—a chorus of them—playing one part.
Chorus pedals accomplish this by splitting your signal into multiple “voices,” modulating their pitch and timbre, and delaying those voices slightly from the main signal. The result is a full and shimmery ensemble of “voices,” simulating the sound of multiple instruments playing the same part.
The number of tone-shaping knobs you get with a chorus pedal depends on which pedal you’re using, but most of them feature depth, rate, and tone controls that let you adjust how dramatic you want the effect to be.