interpret following C statement?

hello,
how to interpret following C statement?
#define TCP_SKB_CB(__skb) ((struct tcp_skb_cb
*)&((__skb)->cb[0]))
what is use of tcp_skb_cb structure in kernel source tcp.h? how
TCP_SKB_CB(skb) is translated in its definition? regards,
linux_lover

__________________________________________________
Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around
http://mail.yahoo.com