关于XML，序列化的问题

private void writeDocument() throws IOException{
FileOutputStream fileStream=new FileOutputStream(logFile);
Serializer writer=new Serializer(fileStream);
writer.setIndent(2);
writer.write(document);
}
请问Serializer，序列化是用来干什么的？
writer.setIndent(2);
writer.write(document);这两句又是什么意思，谢谢！
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