A “typeout frame” is a special Emacs frame which is used instead of the echo area (minibuffer) to display messages from SLIME commands. This is an optional feature. The advantage of a typeout frame over the echo area is that it can hold more text, it can be scrolled, and its contents don't disappear when you press a key. All potentially long messages are sent to the typeout frame, such as argument lists, macro expansions, and so on.
If the typeout frame is closed then the echo area will be used again as usual.
To have a typeout frame created automatically at startup you should
slime-typeout-frame package. (see Loading Contribs.)
slime-typeout-frame-properties specifies the
height and possibly other properties of the frame. Its value is
make-frame. (see Creating Frames.)