El Niño is a cyclical pattern of warming and cooling over the mid and south Pacific (the cooling phase is called La Niña). Cycles last between a couple of years and a decade.
As El Niño is the warming phase, warming overall global temperatures make it more extreme and unpredictable. Since the Pacific is massive, El Niño has a massive effect on weather patterns in a big chunk of Asia, Oceania, and the Americas, which then has global knock-on effects.
It's a natural climate phenomenon that stems from the Earth's geography, but like all weather and climate patterns, human activity effing with the balance so much over such a tiny sliver of geological time can have a great deal of impact on how powerful and/or disruptive it is.