What’s this bar under my keyboard?
It appears to be the 'home' button/swipe area for the iPhone X.  Because there is no Home button, Apple added a section of the screen for swiping up to access 'Home' as well as unlocking the iPhone.  

It will have no use to you or anyone else who isn't on an iPhone X.  As clarified in the comments by @grgarside, the fact that you can see it at all (on a non-iPhone X) is due to a bug in the iOS 11.1 beta.

Here's an image of the iPhone X and you can see the Home bar at the bottom of the screen.