It's a boss area in Elden Ring. They're saying every step you take in Bloodborne has a purpose compared to wandering around an openworld setting aimlessly and kind of hoping you're going to an appropriate area.
Most games have such areas but those are spread around the map. So you have 'normal dungeon' where you deal with average mooks (often randomly generated these days) and then you end up in a legendary dungeon where you need to kill 'the boss'.
I'm assuming Bloodborne is just bossfight after bossfight.
I interpreted it to mean a primary, necessary, and unique area/dungeon, as opposed to an optional side dungeon with reused assets or a nonspecific "overworld" so to speak. Like if stormveil castle linked directly to Raya Lucaria which linked directly to volcano manor, and so on, without an open world to traverse in between.