The StreamReader is a class in the System.IO namespace of the C# programming language, designed for reading characters from a byte stream in a specific encoding. It is commonly used to read text files efficiently, allowing developers to process large amounts of data without loading the entire file into memory at once. Using StreamReader, you can read data line by line or character by character, making it versatile for various applications. It also provides methods for reading and closing the stream, ensuring that resources are managed properly. Overall, StreamReader simplifies file handling in C# applications.