The function scanf is a standard input function in the C programming language used to read formatted data from the keyboard. It allows programmers to capture user input by specifying the type of data expected, such as integers, floats, or strings, using format specifiers like %d, %f, and %s. When using scanf, the user must provide the address of the variable where the input will be stored, typically using the address-of operator (&). This function returns the number of items successfully read, helping to verify if the input was processed correctly.