The line at the bottom of the frame (below the mode line) is the echo area. It is used to display small amounts of text for various purposes.
Echoing means displaying the characters that you type. At the command line, the operating system normally echoes all your input. Emacs handles echoing differently.
Single-character commands do not echo in Emacs, and multi-character commands echo only if you pause while typing them. As soon as you pause for more than a second in the middle of a command, Emacs echoes all the characters of the command so far. This is to prompt you for the rest of the command. 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. You can change this behavior by setting a variable (see Display Custom).
If a command cannot be executed, it may display an error message in the echo area. Error messages are accompanied by beeping or by flashing the screen. The error also discards any input you have typed ahead.
Some commands display 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. Sometimes the sole purpose of a command is to show you a message giving you specific information—for example, C-x = (hold down <CTRL> and type x, then let go of <CTRL> and type =) displays a message describing the character position of point in the text and its current column in the window. Commands that take a long time often display messages ending in ‘...’ while they are working, and add ‘done’ at the end when they are finished.
Echo-area informative messages are saved in an editor buffer named ‘*Messages*’. (We have not explained buffers yet; see Buffers, for more information about them.) If you miss a message that appears briefly on the screen, you can switch to the ‘*Messages*’ buffer to see it again. (Successive progress messages are often collapsed into one in that buffer.)
The size of ‘*Messages*’ is limited to a certain number of lines.
The variable message-log-max specifies how many lines. Once the
buffer has that many lines, each line added at the end deletes one line
from the beginning. See Variables, for how to set variables such as
message-log-max.
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. When the minibuffer is in use, the echo area begins with a prompt string that usually ends with a colon; also, the cursor appears in that line because it is the selected window. You can always get out of the minibuffer by typing C-g. See Minibuffer.