Mono sum collapses stereo left and right channels into one signal; phase describes whether those channels are time-aligned, affecting how they combine.
Mono sum is the process of combining a stereo audio signal's left and right channels into a single mono channel. Phase refers to the time relationship between those two channels: when they are in phase (aligned), their waveforms add together; when they are out of phase (inverted or offset in time), matching frequencies partially or fully cancel each other when summed.
Why it matters
Club sound systems often run mono or near-mono, and venue acoustics can effectively mono-sum the output across the room. If a DJ's source audio or processing introduces phase issues, bass and low-mid frequencies can disappear or thin out significantly in the venue even though the mix sounded full in headphones.
In practice
Use a mono sum button on your mixer or a phase correlation meter in your software to check your mix in mono before or during a set. If bass collapses, look for heavily stereo-widened low frequencies in your tracks or send/return processing that is introducing phase offset.
