The Second Realm is extremely hard to define to those who do not thrive inside it. It is the meeting place between the glory and beauty of the First Realm and the amazing infinity of the Third Realm. The few native creatures from this realm, such as the fae, muses, and Gods are made of dream energy provided by ningen, and this realm is the source of all power in the multiverse. Traditionally, no creature can enter the Second Realm without dreaming. The Second Gate is used for this purpose, but its location is unknown but to a few. The Second Realm is everywhere outside of dreams as well. It is creativity incarnate, it is the spirit that fills inanimate objects and gives everything life. The Aeternalae use the Second Realm frequently as a tool, though often without understanding its mechanics, as it is part of their heritage and what even causes the Influence. The Second Realm's power isn't always dramatic or flashy: sometimes it is subtle and beautiful. If a writer or artist has claimed they were moved by an unseen force, this is it (see: Magic).
The Second Realm is also the "ozone" of the First Realms, keeping a strict limit on how many Youkai are allowed in and out of them. If there isn't enough "conceptual" energy, then the Youkai is refused entry. This means if a First Realm does not have guns, the TIA Military Guild would be refused entry. The Second Realm is also the reason Etheriginae or other Youkai can't summon reinforcements endlessly. Without the Second Realm, the Third Realm would become as lifeless as a math formula, and the First Realm would become unsafe with Youkai flow unchecked and Ningen would be without dreams or aspirations.
The Second Realm is the only other realm that First Realm beings enter, if fleetingly. There are a few exceptions to this rule, being the more spiritually inclined tribal peoples who have learned long ago how to dreamwalk properly...