What does the “rep stos” x86 assembly instruction sequence do?

Question

I recently stumbled across the following assembly instruction sequence:

rep stos    dword ptr [edi]

Answer 1

Empty array: 
char buff[256] = { }; 

 776      1c5:   48 8d 95 e0 fc ff ff    lea    -0x320(%rbp),%rdx
 777      1cc:   b8 00 00 00 00          mov    $0x0,%eax
 778      1d1:   b9 20 00 00 00          mov    $0x20,%ecx
 779      1d6:   48 89 d7                mov    %rdx,%rdi
 780      1d9:   f3 48 ab                **rep stos %rax,%es:(%rdi)**