When you enable unison on a synthesizer, it plays the same note multiple times simultaneously with each voice slightly detuned and offset in the stereo field. Two voices sound like a gentle chorus; eight voices sound like a full ensemble.
The main parameters are voice count (how many simultaneous copies), detune (pitch spread in cents between voices), and stereo spread (how wide the voices are panned). FL Studio's 3xOsc, Serum, and Vital all offer unison controls with these parameters.
Unison adds CPU load because each voice is processed independently. For a supersaw sound in Serum or similar: enable 7-voice unison, set detune to 0.20–0.35, enable stereo spread, and choose a sawtooth waveform. Filter out below 100–150 Hz to prevent bass muddiness.