What is \\1 in regular expressions? [duplicate]

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• Reference - What does this regex mean? 1 answer

What is the (\\1) in this case? And how does the method "group" work with it?

public class Test{
      public static void main(String[] args){  
        Pattern p = Pattern.compile("(something).+(\\1)");  
        Matcher m = p.matcher("lol asd adlso something ALALALALA something boba" );  
        if(m.find())
            System.out.println(m.group());    
      }
}