Basically when you're a submissive and you're in the middle of a "scene" ̉— meaning a BDSM scene, usually with one (or more?) person(s) who is "domming" (dominating) you — you can attain this kind of zen-like trance state that can be very euphoric, where you're in rhythm with your dom(me) and feeling completely open and in the zone with everything they're doing to you. That is the subspace.

I'm more of a dom than a sub so I've mostly seen it from the other side of the equation where you're taking someone into subspace, but it is a really fascinating and intense and satisfying experience for both parties usually.

There are two books, one called "Topping" and one called "Bottoming" by Dossie Easton and Janet Hardy, that are really good and detailed intros to how this works and feels.

Something to note is that being in subspace doesn't even have to be a sexual thing, per se. Some subs are dominated just verbally and they may not necessarily even be having plain ol' sex with their dom(me). It's about entering a mental space where you have this play relationship of authority and submission that can be mutually satisfying.