When browsing an Onion Service, Tor Browser displays different onion icons in the address bar indicating the security of the current webpage.

An onion means:

• The Onion Service is served over HTTP, or HTTPS with a CA-Issued certificate.
• The Onion Service is served over HTTPS with a Self-Signed certificate.

An onion with a red slash means:

• The Onion Service is served with a script from an insecure URL.

An onion with caution sign means:

• The Onion Service is served over HTTPS with an expired Certificate.
• The Onion Service is served over HTTPS with a wrong Domain.
• The Onion Service is served with a mixed form over an insecure URL.