This warning appears on Resilio Sync WebUI page over HTTPS protocol. It's not critical and does not bear any threats. It appears because Resilio Sync uses a self-signed certificate for WebUI, that browsers do not recognize.
Click on the warning icon -> Certificate to view the certificate itself. Make sure that it's indeed Resilio's, and proceed with one of the steps below to resolve it
Ignore the warning
Click on "Advanced" (depending on browser, it might be named differently) and select to "Proceed to <page> (unsafe)".
Clear HSTS cache
If there is no option to "Proceed", as a quick fix just type
thisisunsafe on your keyboard and the browser will let you in instantly. Works for Chrome-like browsers.
For permanent fix, you need to clear HSTS cache so that browser uses HTTP protocol for the WebUI. Depending on the browser, the steps will be different. Refer to this guide or any other of your choice to clear the HSTS settings from your particular browser.
Use your own trusted certificate
If you have your own web certificate, run Sync in config mode, providing the path to the created certificate and private key in the configuration file:
,"ssl_certificate" : "/path/to/cert.pem"
,"ssl_private_key" : "/path/to/private.key"