Orca is a Linux screen reader. Since it is part of the GNOME project it should come preinstalled with Ubuntu installations.
To turn on the screen reader you can either go to Settings > Accessibility and then activate Screen Reader in the "Seeing" section or you can simply type orca in your terminal.
Note
It may feel quite strange in the beginning to use a screen reader. It is constantly commenting on everything you do and every application you visit will be read aloud.
Once you started your screen reader you can simply navigate to whatever application you want to test and the screen reader will start to do its thing. If your screen reader isn't starting to read out the content, you may need to restart the application. If that still does not work, then the application you want to test may not be supported. Since we are usually building websites though, that should not pose a problem.
If you wish to customize your screen reader you can do so by typing orca -s into the console. That should open a settings window where you can adjust your screen reader. I would recommend turning down the "Rate" in the "Voice" tab as the screen reader is reading things quite fast.
Handling
I have noticed, while testing my application, that the screen reader sometimes behaves a bit weirdly if you are using the mouse to navigate the page. It would swallow sentences that I would expect to be read out. That did not happen when I was using the keyboard though, which should be the expected tool anyway.
