Why in the flying fucks are you dealing with PIs if you're an HTML beginner? I've been working with web for quite some time and I've never heard of those before.
Processing Instructions work layers below HTML; they are used at the parser level. In short, when you're designing a parser for a specific flavor of xml, there may be some information that you'd like for some application to use and run.
For example, let's say that you're working with a XML that defines the download information for a set of torrent files. You'd like that as soon as the xml is parsed, the files begin downloading. A PI will signal the parser to open an external program with specific data (in this case, the torrent client with a magnet link).
This is an extremely high level example. Usually, PIs are used to add stylesheets to an XML document.
Still, unless you're writing a parser yourself, I see no reason as to why you should be dealing with those.
