The effect is similar to a “chorus” playing a unison together; i.e. they start at different times and have subtle differences in tuning. 

Chorusing is a slightly elaborated version of doubling. A signal is delayed approximately 15-35 milliseconds and mixed with the undelayed signal. The delay time is modulated by a low-frequency-oscillator to achieve a shimmering effect due to a combination of beat-frequencies and the slight pitch-bending that occurs as the delay time is changed.  
Thus chorusing describes what occurs when (say) several clarinets play in unison: They do not start at the precisely the same time and they have slight pitch/vibrato variations.