Are you tired of people looking at your phone screen? Do you think it's too bright at night even at the lowest setting? 
Then Dimmer might be the right app for you. It is free and open-source and available on the F-Droid store.
The app itself is easy to use and configure. Once you start it, you will get a black overlay on top of your screen that will lower it's brightness. You can configure how bright you want it to be from the notification that the app provides. This is perfect for using a smartphone while in transit, as it keeps the screen bright enough for you to see, but not bright enough for people around you to peek. Once you're done with using the app, you can simply click on the notification button to close it or launch it again.