What does this ms outlook red colored badge (icon) mean?

What does the red coloured badge mean?

What do the Outlook icons mean? lists all of the icons in outlook and explains what they mean.

The icon you are referring to is in the Security section and indicates that the message has a Valid Signature:

If you Open the message and then click on this icon you can find more information about the digital signature.

See View a digital signature in a signed e-mail message for more information.

Source What do the Outlook icons mean?

It indicates that the message was digitally signed.