It means that Google is experimenting with using Rust to build parts of Android. That could mean parts of the OS, tests, tools, SDK, etc...
As a simple example of why this isn't necessarily super interesting is Python has been in the Android tree for ages, but just for some build & test stuff.
But maybe it's interesting just in that the Google engineers working on Android have started playing with Rust enough to bother checking it in to the tree.
Pretty much what Mozilla did when Rust got more serious.