The function strcat is a standard library function in the C programming language used to concatenate two strings. It takes two arguments: the destination string and the source string. The destination string must have enough space to hold the combined result, as strcat appends the source string to the end of the destination string, modifying the destination in place. When using strcat, it is important to ensure that the destination string is properly null-terminated and has sufficient allocated memory. If not, it can lead to undefined behavior, such as buffer overflows. This function is part of the string.h header file, which contains various string manipulation functions.