This addon contains signatures of the expected content at https://www.airbornos.com/. If the signatures don't match the content received from https://www.airbornos.com/, this might mean an attacker, such as a MITM, the webhoster or the website owner might have changed the website's content for everyone or especially for you.
The signatures are tied to the website's current certificate, so you aren't locked out of the website when the website is update but the addon is not. (Although this means that if the addon is not updated, you might no longer be protected.)
In the future, certificates created by the website owner will be tracked by Certificate Transparency. Together, Certificate Transparency and HTTPS Content Signatures will ensure that everyone receives the same content when they go to websites that use both, and that changes to that content can be tracked.
This addon also supports containing signatures directly in the website's certificate, but no websites support that yet.