The Relationbase is a database of all the characters and their relationships with each other. This is useful for establishing pre-existing connections and friendships between characters whose players don't necessarily know each other. This avoids the awkward in-character introduction between characters who would likely have met prior actual gameplay - for example, if these characters were in the same House back at Hogwarts or if they work in the same department at the Ministry. Additionally, this provides a good way to build interesting backstory between characters.
Having a thread in the Relationbase is entirely optional but very useful. Simply create a 'New Topic' with your character's full name as the topic title. Most players then include a "legend" that categorizes all their possible relationships from acquaintances to enemies to lovers. Then players can post in other characters' threads and build their network.
If you have any questions, please post them below as always.
Cheers,
MM