Can someone give an explanation of what XML is?

XML stands for eXtensible Markup Language

It basically defines the "pattern" that HTML is based on, where things are identified using tags, and tags can be nested, and tags can have attributes.

XML is more generic than HTML, where the tags in HTML come from a defined set, and each tag has a particular meaning to the document. But in XML the tags and attributes can be anything.

It's used in web and other applications primarily as a way to store and transmit data. It has an advantage over other data formats in that it can be read by humans and edited with ordinary text editors.

Here's an example of XML (but this is not valid HTML):

<food>
    <fruit name="banana">
        <color>Yellow</color>
        <shape>Long</shape>
        <taste>Sweet</taste>
    </fruit>
    <fruit name="orange">
        <color>Orange</color>
        <shape>Round</shape>
    </fruit>
    <nut name="almond">
        <color>Brown</color>
    <nut>
</food>