according to the dictionary, an irregularly occurring and complex series of climatic changes affecting the equatorial Pacific region and beyond every few years, characterized by the appearance of unusually warm, nutrient-poor water.

 the climate phenomenon that occurs when a vast pool of water in the eastern tropical Pacific Ocean becomes abnormally warm. Under normal conditions, the warm water and the rains it drives are in the western Pacific.

El Nino is most direct impacts are droughts in normally damp places in the eastern Pacific, such as parts of Indonesia and Australia, while normally drier places like the west coast of South America suffer floods.

Typically, it comes around every five years and what usually happens is that warming in the oceans caused by the winds leads to diffusion of this warming all over the globe. It changes atmospheric pressures with consequences for rainfall, wind patterns, sea surface temperatures and can sometimes have a positive, and sometimes a negative effect on those systems

https://www.eh-resources.org/el-nino-bibliography/