Cortex is a running JATOS server where you can try out JATOS in the Internet instead of your local computer:
https://cortex.jatos.org (log in with email@example.com and abc1234)
This is a normal JATOS installation with many example studies already imported. You can run the examples or import your own studies if you want to test them in a JATOS running in an online environment.
But be aware that every day at 8:00 UTC this JATOS server will be reset to its inital state and all changes will be lost. In other words this JATOS is not meant to run your studies online.