What does \1and \( \) in sed do ?

Today , when I learned about  sed,I found a question there.That is :
$ cat hello 
   hellohellohello 
 I want the output like this :
hello 
hello 
hello 
So the code is this :    sed 's/\(hello\)/@\1/g' hello | tr "@" "\n" 
So what's the meaning of this?
 让hello与hello之间用特殊字符(在这里我用的是@)进行连接,然后在把该特殊字符(@)用tr命令转化为"\n"(换行符).
this is the explain.
It's a little hard to understand .So I search google ,found this :

Here's a simple example:

$ echo 'abcabcabc' | sed 's/\(ab\)c/\1/'
ababcabc
$ echo 'abcabcabc' | sed 's/\(ab\)c/\1/g'
ababab
$ echo 'abcabcabc' | sed 's/\(ab\)\(c\)/\1d\2/g'
abdcabdcabdc

So ,the meaning of \1 is the first part of s/ /,
in  sed 's/\(hello\)/@\1/g' hello | tr "@" "\n"  ,\1 refers to \(hello\)
in 

echo 'abcabcabc' | sed 's/\(ab\)\(c\)/\1d\2/g'  ,

\1d refers to \(ab\)   ,  \2 refers to \(c\)

\(...\) would capture the characters specified inside of the parens and \1 would be used to reference the first match, this is a part of regex.



I think it's helpful, so I note it here.thanks