It's basically invisible automation for creating and renewing LetEncrypt certs on App Engine.
The traditional process for installing a custom domain SSL cert on App Engine was very clunky. Involved running OpenSSL commands, cut-n-pasting PEM data, etc. If you were using LetsEncrypt, then it was more or less impossible to automate... you had to go through a tedious manual process every 3 months (including updating your app, to respond to the LetsEncrypt verification endpoint!).
A click on the browser padlock icon says that it's a LetEncrypt cert. Unsurprising, since Google is such a major sponsor. Also unsurprising that Google chooses to focus on their own branding rather than call attention to it.
The traditional process for installing a custom domain SSL cert on App Engine was very clunky. Involved running OpenSSL commands, cut-n-pasting PEM data, etc. If you were using LetsEncrypt, then it was more or less impossible to automate... you had to go through a tedious manual process every 3 months (including updating your app, to respond to the LetsEncrypt verification endpoint!).