What restrict is in strcpy_s and strcpy

In https://en.cppreference.com/w/c/string/byte/strcpy Signature of methods is listed as

char *strcpy( char *dest, const char *src );
(until C99)
char *strcpy( char *restrict dest, const char *restrict src );
(since C99)
errno_t strcpy_s(char *restrict dest, rsize_t destsz, const char *restrict src);
(2) (since C11)

What is this restrict word/keyword?