You see messages when you enter and exit from configuration mode, when you commit a configuration, and when you type a string or value that is not valid.
When you commit a configuration, the JUNOS software checks the configuration you are committing. If there are no problems, a message indicates that the configuration was accepted. If there are problems, a message indicates where the errors are located.
In the top-level CLI commands and in configuration mode, if you type an invalid string—for example, if you type the name of a command or statement that does not exist—you see the message "syntax error" or "unknown command." A caret (^) indicates where the error is. Examples:
user@host>clear route^ syntax error, expecting <command>. [edit] user@host#telnet^ unknown command.
When the number of choices is limited, a message might display the commands you can enter to correct the syntax error. For example,
[edit]
user@host# load myconfig-file<Enter>
^
syntax error, expecting `merge', `override', or `replace'.