If anyone has questions about how this came to be... dont ask. Watch the video series from Comic's Explained. He does a great job of making sense of all the nonsense.
So some important information;
-The end of the Marvel multiverse is caused by the colliding of worlds (two world's of different universes)
-Worlds are colliding due to "incursions" or disruptions in the space time continuum
-Ultimately it was discovered that these incursions were intentionally being caused by single person. This person was a complete mystery to those trying to stop the destruction until after months and issues worth of storylines it was found to be Dr Doom (616), hoping to use the resulting energies for his own
-He achieved this by killing off the Molecule Man of different realities (apparently he was a dimensional constant in the multiverse, the result of an experiment by the Beyonders, whom were also hunting Doom)
-The world (Earth or any planet experiencing an incursion, but I think only Earth's are) can be spared from destruction by destroying the opposing realities planet of the same name. If the opposing planet is destroyed, both universes are spared destruction with only one planet being sacrificed (Namely Earth).
-the earths in question are 616 (main Marvel) and the Ultimate universe. One is not destroyed, and both earth's and the remainder of the Marvel multiverse are wiped from existence.
-due to the construction of a "life raft" by Reed Richards, close to a dozen 616 characters survive the end including; Reed, Black Panther, Spiderman, Captain Marvel, Star Lord, Cyclops, and Thor (female).
Feel free to add and elaborate!