The minibuffer is the facility used by Emacs commands to read arguments more complicated than a single number. Minibuffer arguments can be file names, buffer names, Lisp function names, Emacs command names, Lisp expressions, and many other things, depending on the command reading the argument. You can use the usual Emacs editing commands in the minibuffer to edit the argument text.
When the minibuffer is in use, it appears in the echo area, and the terminal's cursor moves there. The beginning of the minibuffer line displays a prompt in a special color, to say what kind of input you should supply and how it will be used. Often this prompt is derived from the name of the command that the argument is for. The prompt normally ends with a colon.
Sometimes a default argument appears in parentheses before the colon; it too is part of the prompt. The default will be used as the argument value if you enter an empty argument (that is, just type <RET>). For example, commands that read buffer names always show a default, which is the name of the buffer that will be used if you type just <RET>.
The simplest way to enter a minibuffer argument is to type the text you want, terminated by <RET> which exits the minibuffer. You can cancel the command that wants the argument, and get out of the minibuffer, by typing C-g.
Since the minibuffer uses the screen space of the echo area, it can conflict with other ways Emacs customarily uses the echo area. Here is how Emacs handles such conflicts: