The function strncpy is a part of the C programming language's standard library, specifically included in the header file string.h. It is used to copy a specified number of characters from one string to another. This function helps prevent buffer overflows by allowing the programmer to limit the number of characters copied, making it safer than the simpler strcpy function. When using strncpy, the programmer must specify the destination string, the source string, and the maximum number of characters to copy. If the source string is shorter than the specified length, strncpy will pad the destination string with null characters to ensure it is properly terminated.