The JShell REPL commands work give you a way to manipulate failed, overwritten, dropped and start-up snippets and even other commands. All commands start with a forward slash (/).
For those new to JShell, snippets are little chunks of Java code. For a quick introduction to JShell, see my Java 9 REPL tutorial. Once you get accustomed to JShell, you will realize there is not much of a limit to what you can do with it.