A stereo recording places sounds between two channels. Sounds identical in both channels appear in the centre, while differences between left and right create a sense of space. Engineers often think in terms of mid and side: the mid is what the channels share, the side is what differs. With no side signal the result is mono; more side makes the image wider, but too much leaves a hole in the middle.
Widening tricks such as very short delays between channels can sound impressive on headphones yet partly cancel when the mix is summed to mono on a single speaker. A common practice is to keep bass and kick centred, for instance by filtering low frequencies out of the side signal. In jazz lo-fi, gentle width on keys and reverb helps the music feel roomy on headphones while still holding together in mono.