`std::runtime_error` is a standard exception class in C++ that is used to indicate errors that occur during the execution of a program. It is part of the C++ Standard Library and is included in the `<stdexcept>` header. This exception is typically thrown when a program encounters a problem that it cannot handle, such as invalid input or resource allocation failures. When a `std::runtime_error` is thrown, it can be caught using a try-catch block, allowing developers to manage errors gracefully. The class provides a constructor that accepts a string message, which can be used to describe the specific error, making debugging easier.