The function strlen is a standard library function in the C programming language used to determine the length of a string. It counts the number of characters in a string, excluding the null terminator, which marks the end of the string. The function returns an integer value representing this length. To use strlen, you need to include the header file <string.h> in your program. The function takes a single argument, which is a pointer to the string, and it returns the length as an integer. This is useful for various tasks, such as validating input or allocating memory for strings.