IMPORTANT UPDATE: This article has been slightly updated as of the prerelease of Magic 2014 on July 13; Please refer to the bolded revisions below.
Genesis Wave is one method to getting multiple cards into play at the same time. The higher the value of X, the more cards you look at that the ‘bigger’ the things you are allowed to put into play. One important thing to keep in mind with regards to Genesis Wave is that all of the cards that are chosen to be put into play enter play at the same time. They do not enter play in any sequential order one after another.

This means that multiple enters the battlefield triggered abilities can trigger at one time. For example, if when resolving a Genesis Wave where X equaled five, you revealed four lands and a single Ob Nixilis, the Fallen, the ability of this Ob Nixilis will trigger four times, once for each of the lands that came into play at the same time as Ob Nixilis did. If instead of 4 lands and a single copy of Ob Nixilis, three lands and two copies of Ob Nixilis were to come into play, then we’d have even more triggers. All of these permanents would enter the battlefield, and the ability of each Ob Nixilis would trigger three times each, for a total of six triggers. You would then choose one copy of Ob Nixilis to keep, and the other would be put into the graveyard as a State-Based Action, then those six triggers are put on the stack. The Ob Nixilis you keep will end up getting nine +1/+1 counters (three from each of his three triggers), and your opponent would lose 18 life (he still loses life from the triggers of the dead Ob Nixilis, but that Ob Nixilis won’t gain counters).