The original Secret Wars (1984) was an event in which a cosmic entity known as the Beyonder kidnapped heroes and villains from Earth in order to watch that battle for his entertainment.

Secret Wars (2015) was an event in which the entire multiverse was being destroyed by the entire race of Beyonders (the 1984 Beyonder was a child version of these) who live in the void outside of the multiverse. The short version is basically that universes were collapsing into each other and the final two were the main marvel universe (616) and the ultimate universe (1610). In order to save the multiverse Dr Doom used The Molecule Man, who was kind of an Multiversal bomb designed by the Beyonders, to kill them and steal their power for himself. Upon achieving this, Doom saved the fragments of the universes that he could and knitted them together in a new Battleworld with himself as God King. The survivors of the 616 and 1610 universes joined forces to undermine Doom's authority and launch a final battle to take his power. There's about 10 years of detail that I've missed out but that's about the gist of it.