According to UESP, the Compact was agrees on the 5th of Rain's Hand, 1E 2920, the last year of the First Era, after Molag Bal manifested and destroyed the Wood elf village of Gil-var-delle, which you can actually visit in Grahtwood.
No mortal knows the exact terms, even into the Fourth Era. In broad strokes, it seems to limit most physical manifestations and interactions on Nirn of the Daedric Princes that agreed to it, originally eight (Azura, Boethia, Hermaeus Mora, Mehrunes Dagon, Hircine, Malacath, Morag Bal, and Sheogorath) with Mephala and Clavicus Vile agreeing at the end of the Summerset story. That leaves Nocturnal, Vaermina, Peryite, Namira, Sanguine and Meridia as Non-signatories unbound by it (which would explain things like Nocturnal and Sanguine showing up in-person in Skyrim.) Mehrunes Dagon tried to break the compact almost immediately later in 1E 2920 to sack Mournhold, only to get curbedstomped by Almalexia and Sotha Sil.
The only clue as to how Sotha Sil managed to get these princes to agree to the original compact is Secrets Overheard in Apocrypha, which might also explain why events such as the Planemeld and the future Oblivion Crisis are possible: Sotha Sil apparently offered *something* of such value to Hermaeus Mora that he agreed to a pact with all of Nirn to not interfere. But that this pact goes both ways, and mortals are technically not suppose to visit or go romping around in their realms of Oblivion either. It may be that while the Daedric Princes who have agreed to the Compact cannot mess with Nirn directly of their own volition, the actions of mortals could be the loophole they need to still mess with Tamriel, especially when Akatosh's own deal with St.Alessia to protect Nirn is in limbo when the Dragonfires aren't lit. After all, every Dolemen event begins with Worm cultists preforming a sacrifical ritual to Molag Bal. With regards to Dagon and the Oblivion Crisis, it could also be that with the Tribunal all dead or missing after the events of Morrowind and its expansions, there is noone left to enforce the terms.