The MiniFinder debuted with Macintosh System 2.0 in April 1985. Some consider it to be a predecessor to At Ease.
To use it, you would choose Use MiniFinder from the Special menu in the Finder after selecting the applications you wished to use in a Finder window.
When this was all done, you would return to the MiniFinder instead of the ordinary Finder whenever you quit an application. Applications made available to MiniFinder could be launched by double-clicking the icon.