What does this icon mean in VS2019?

This icon shows the inheritance chain/margin and was added in version 16.10 of Visual Studio. From the 16.10 release notes for Visual Studio:

There is now a visual representation for navigating and inspecting the inheritance chain. This option is off by default so you will need to turn it on in Tools > Options > Text Editor > C# > Advanced and select Show inheritance margin. Enabling inheritance margin will add new icons to the margins representing your code’s implementations and overrides. Clicking on the inheritance margin icon will display inheritance options that you can select to navigate to.

Sure enough, if you navigate to the option in the tools menu, it is at the bottom:

I would check your settings for this - it should be off by default, meaning somehow it was turned on for you. The icon should show the inheritance chain if you click on it. If it isn't, perhaps turn the option off and back on, or restart Visual Studio (it's experimental apparently, so it could be bugged).