The delay function in C++ is used to pause the program for a specified amount of time. It is part of the library and is implemented using the function "sleep" or "this_thread::sleep_for" depending on the version of the C++ standard being used. The delay function takes a single argument, which is the duration of the pause in milliseconds. When the delay function is called, the program will pause execution for the specified duration before continuing with the next instruction. This can be useful for creating time delays in programs, such as in animations or simulations, or for controlling the timing of certain operations. However, it's important to use the delay function judiciously, as excessive use of time delays can lead to unresponsive or inefficient programs.