System.Net.Sockets is a namespace in the .NET framework that provides a set of classes for network communication. It allows developers to create applications that can send and receive data over the internet or local networks using the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). The classes in System.Net.Sockets enable functionalities such as establishing connections, sending messages, and handling data streams. This makes it essential for building client-server applications, online games, and any software that requires real-time data exchange over a network.