What does "abs:%s" and (tag.indexOf("[") + 1) means and do?

I am using Jsoup in my project and i am try to get understand what these lines of code in my HTMLparser.java is step by step doing:

static List<LinkNode> toLinkNodeObject(LinkNode parentLink, Elements tagElements, String tag) {
    List<LinkNode> links = new LinkedList<>();
    for (Element element : tagElements) {
        if(isFragmentRef(element)){
            continue;
        }

        String absoluteRef = String.format("abs:%s", tag.contains("[") ? tag.substring(tag.indexOf("[") + 1, tag.length()) : "href");
        String url = element.attr(absoluteRef);

        if(url!=null && url.trim().length()>0) {
            LinkNode link = new LinkNode(url);
            link.setTag(element.tagName());
            link.setParentLink(parentLink);
            links.add(link);
        }
    }
    return links;
}

Can you tell me what this line is doing ?

 String absoluteRef = String.format("abs:%s", tag.contains("[") ? tag.substring(tag.indexOf("[") + 1, tag.length()) : "href");