This is jshell, Java's REPL (Read, Eval, Print Loop). Type a Java expression at the jshell> prompt on the right, hit return, and it evaluates it and prints the resulting value — no class, no main, no compiling. You can declare variables and methods and then use them in later expressions:

jshell> 6 * 7
$1 ==> 42

jshell> String shout(String s) { return s.toUpperCase() + "!"; }
|  created method shout(String)

jshell> shout("hello")
$2 ==> "HELLO!"

It's a real terminal: Tab completes names, the arrow keys edit the line and walk your history, and Ctrl-C cancels the line you're typing — or interrupts a snippet that's stuck running forever, without losing your session. Restart gives you a completely fresh session. Type /help for jshell's own commands.

The editor on the left works with jshell: click to load what you've written into the session — a Loaded from the editor: line lists what came in. Your code loads as written: methods written at the top level (no class needed!) are callable directly, while a class loads as a class, so call its methods like Scratch.method(...). Either way, a program's main runs as just main(). Click it again after an edit and the definitions update in place. And if you write a complete program, the Terminal tab's compiles and runs it from main instead.