what does in-line expressions in Appian mean?

Hi , In older versions we had capability to write expression while configuring forms in user input task. Now that has been removed in latest version.

Similar to what Gopalk already said - in older versions you could basically hardcode some interface code into a special expression editor box at the Start Form or User Input Task form configuration dialogs - instead of selecting an interface that was created and saved via the interface editor.  There was almost nothing useful about this, with the sole exception of being able to make a quick-and-dirty "one button" start form or other things like that, which of course was a hacky workaround anyway.

Whilst on the topic, you're able to provide "inline expressions" in various places in Appian (Scrip Tasks, Record Action Visibility etc etc). But it's bets practice to encapsulate anything bigger than a trivial piece of code as it allows you to maintain and test it independently of the object it's used in and often it leads to reusable components, which again reduces the maintenance burden.