It's a school of economics that became dominant in the 60s, it emphasises free market dynamics, much like the classical school, and especially focuses on consumption and optimizing economic actors, these actors are a very simple (and unrealistic) model of consumers. They also neglect production as an aspect of the economy.

Fun fact: The neoclassical economic school managed to remove the word "political" from "political economy" at the turn of the 20th century.