A mono mix is when your music, recordings, or any other type of audio is played at equal volumes out of both speakers (no panning), meaning the listener won't experience two separate channels but one single channel (called the center channel or mono channel) in the center of the stereo field.
Should I mix in mono?
You should mix in mono before you add any EQ, compression, or other plugins. And you should do this with every mix if you want an industry-standard track. And your DAW will have a mono button, possibly on or near the master track. Just hit that and start mixing!
Why mixing in mono is the secret?
When we mix in stereo we can separate mix elements out in the stereo field to make them easier to hear. When we collapse the mix to mono, these different elements start to obscure each other again. The fact is, that if your mix isn't clear and punchy in mono – it just “isn't ready yet”.
Can you mix in mono in headphones?
For mono sources, mixing in mono is a great way to start, then finish it off with stereo. Stereo sources don't mix well in mono, as half the signal is gone. And some stereo sources (like synth unisons) always cause problems like phase cancellation in mono.