XML supports something called Mixed Content. It looks like this:

<letter>
    Dear Mr. <name>John Smith</name>.
    Your order <orderid>1032</orderid>
    will be shipped on <shipdate>2001-07-13</shipdate>.
</letter>

For most people (including me), you aren't trying to do Mixed Content but you've accidentally ended up with one or more characters outside of tags in your XML file.

Possible causes:

• Typos
• Invisible characters caused by copying from PDFs or copying from webpages

Further reading: https://www.w3schools.com/xml/schema_complex_mixed.asp