Once echoing has started, the rest of the command echoes immediately as you type it. This behavior is designed to give confident users fast response, while giving hesitant users maximum feedback. If a command cannot be executed, it may print an error message in the echo area. Error messages are accompanied by a beep or by flashing the screen. Also, any input you have typed ahead is thrown away when an error happens.
Some commands print informative messages in the echo area.
These messages look much like error messages, but they are not announced with a beep and do not throw away input. Sometimes the message tells you what the command has done, when this is not obvious from looking at the text being edited. Once the buffer has that many lines, each line added at the end deletes one line from the beginning. The echo area is also used to display the minibuffer , a window that is used for reading arguments to commands, such as the name of a file to be edited. You can always get out of the minibuffer by typing C-g .