Last year (2017) will not be the hottest year on record; it’s likely to be 2nd-hottest. But it will be the hottest which was not enhanced by el Niño conditions.
El Niño is the “warm phase” of a natural oscillation of wind patterns over the Pacific ocean, which increases the amount of heat transferred from ocean to atmosphere. When that happens, there’s more heat in the atmosphere so surface temperatures increase (actually, temperature of the air near the surface, which is what matters for land-based living things). When El Niño subsides, the warming subsides too. The opposite face of the coin is la Niña, when heat tends to go from atmosphere to ocean and surface temperatures tend to be cooler. Together, they make up the el Niño southern oscillation (ENSO).