MapReduce is a programming model for processing and generating large data sets with a parallel, distributed algorithm on a cluster. It consists of two main functions: Map, which processes input data and converts it into key-value pairs, and Reduce, which takes the output from the Map function and merges the key-value pairs into a smaller set of results. This model enables scalable and efficient data processing across multiple nodes, making it suitable for handling big data tasks like indexing, sorting, and data mining. MapReduce is a core component of the Hadoop ecosystem.