The function strcat is a standard library function in the C programming language, used to concatenate two strings. It appends the contents of one string, known as the source, to the end of another string, referred to as the destination. The destination string must have enough space allocated to hold the combined result, including the null terminator. When using strcat, the destination string is modified in place, and the function returns a pointer to the destination string. It is important to ensure that the destination string is properly initialized and large enough to avoid buffer overflows, which can lead to undefined behavior in programs.