It’s one side (the cooler side) of the natural cycle of ENSO (El Niño Southern Oscillation). Basically sea surface temperatures in the central/eastern tropical parts of the pacific swing back and forth from warmer to cooler than average.

This has impacts on large scale weather patterns by impacting things like the available moisture for storms, winds, etc..

Whether we are in La Niña, neutral, or El Niño doesn’t give precise weather forecasts. It just gives a signal to say certain types of events have a higher likelihood of happening.

You can read a little more here.