A security certificate is proof that content is coming from a certain entity.
Certificates are attached to the content in such a way that no else could have authored that information. When the certfied content is presented to someone, the recipient can verify its correctness and authenticity.
Certificates are "signed" by a certificate authority.
The content may be an executable program, a legal document, or a webpage.
For more information about security certificates in the context of the internet, see What are security certificates?.