What it does CL Register in this source code?

Hi all programmer...!
I'm a new programmer with assemblly language and i have a lot of problem...!

Friends, who can describe follow source code for me?!
I can not understand register cl what It does in follow source code!

.section .text 
.globl _start 
_start: 
       nop 
       movl $279, %ecx 
       movzx [b]%cl[/b], %ebx 
       movl $1, %eax 
       int $0x80

Thanks...!