You can pass in a pointer to a time_t object that time will fill up with the current time (and the return value is the same one that you pointed to). If you pass in NULL, it just ignores it and merely returns a new time_t object that represents the current time.
The call to time(NULL) returns the current calendar time (seconds since Jan 1, 1970). See this reference for details. Ordinarily, if you pass in a pointer to a time_t variable, that pointer variable will point to the current time.