Chrome version 66 released this week. With this release, any SSL cert issued before June 1st, 2016 from Symantec, Thawte, Verisign, Equifax, GeoTrust, or RapidSSL are now distrusted by Chrome. This means people browsing your website or Marketo Landing Pages using these certs will get an error in their browser that your pages are not secure! Google has a blog post discussing the changes here Google Online Security Blog: Distrust of the Symantec PKI: Immediate action needed by site operators
If you're still using an older SSL certificate from one of these vendors, you'll need to get new SSL certificates. Marketo's documentation on our SSL implementation process can be found here Overview & FAQ: Secured Domains for Landing Pages . Reach out to your Customer Success Manager on how to get new SSL certificates for your landing pages if you need them.
There's a great blog post here from Courtney Grimes