It is where you have 4 midfielders, broadly in a square / rectangular shape, in the middle of the pitch. Two deep lying two attacking areas. Two 6s and two 10s almost. You give up the wide areas, hence pace at full back/wing back helps.
Man City did it really well season before last at times. It helped drive their possession stats really high. You get a lot of control through the middle where you essentially have 7 or players from back to front. De Zerbi got big success with it when he first landed in this country. Emery at Villa uses it.
Out of possession your two higher lads in the box formation split a bit wider but not a flat 4. They should split the full back and centre back on their side. Basically block the passing angles into the middle, trying to keep them wide. If you do have 3 CBs and wing backs you dont mind crosses into your box, in theory. You dont like them playing through the middle though.
In possession the box shape can morph a bit, but always 4 points of a shape. So box, rectangle, diamond. Always in the middle, rarely out to a wing. You want to try and open space for the wing back. Also trying to get an overload onto their holding player if you can.
It really maximises the chance to control possession and territory. That may not be our best tactic next season, but I think it could be a great one when we target away points. Make teams play round the sides and protect the middle of the goal. When we have it keep it due to numbers.