What does this mean?
This is an HMRC-specific warning. The checker has found the meta tag for the service timeout dialog is sitting outside the head of the page.
Impact on users
There should be no impact on the user, but this can be an indication of other issues in layout implementation. Future updates to the underlying code may only check for the presence of the meta tag in the head in which case this timeout dialog may stop working.
How to fix
Move the meta tag for the timeout dialog into the head of the page.