A SSL certificate establishes a trust relationship (via some complex mathematics) between a client (your web browser) and the server delivering the data.
A certificate is issued by an Certificate Authority (CA), and includes the DNS name of the website. The Certificate Authority is responsible for verifying that the certificate is issued to the correct website administrator.
The website administrator generates a private key - a suitable complex unique number. They use this key to generate a Certificate Signing Request. The CA uses the CSR to build the certificate, and signs it with a number generated from the key and the CA private key.
When you connect to a secure website, the server gives you it’s certificate which contains the website name. Your browser looks at that certificate, finds the issuer (CA), and verifies that the certificate signature was calculated from the Root Certificate of the issuer CA. Sometimes there are intermediate certificates in the chain, but the math is the same - the signature of the website certificate has to correlate with the issuer Root certificate.
Once your browser is happy that the certificate is correct, the browser uses the certificate to encrypt some data to allow the establishment of an encrypted tunnel to pass data through. The server uses the private key to verify the validity of this exchange.
Without this process, you have no verification that your communication is with the site you expect.
Older certificates used the SHA-1 algorithm to generate the cryptographic signature. This has been proven to be weak, and so certificates have been moving to the SHA256 algorithm. Recent browser releases have disabled SHA-1 support, and many sites with SHA-1 only certificates will fail as browsers upgrade.
My recommendation would be to uninstall Chrome, and reinstall the latest version, and retest. I suspect that your certificate store does not contain a SHA256 intermediate certificate for Google’s Root CA. The website still presents both SHA-1 and SHA-256 certificates, but your Chrome cannot verify the SHA-256 certificate via an intermediate, so it uses the SHA-1 cert, but pops up a warning.
It would worry me, but I care about certificates, because I work in that space.