Validation
Validation checks that a HTML document meets the requirements laid out in the W3C specifications. There are multiple different specifications and the Doctype identifies which specification a document conforms too.
Recommendations
Use a on all HTML documents and validate against that Doctype.
Why it matters
Some assistive technology, such as screen readers, may interpret invalid HTML in different ways. There is less risk of accessibility problems arising if the document follows a recognised standard Doctype.
Examples
Recommended:
<!DOCTYPE html>
<html lang="en-GB">
Not recommended:
<html lang="en-GB">
Testing
If validating a webpage against the tool, aim for no errors.