As a Common Lisp programmer, you have probably seen an inspector before. Your Lisp implementation has one, invoked by , but it is probably very basic. SLIME has a widely used one. McCLIM has one as well: Clouseau.
A moldable inspector is customizable. You can add specific views and interactive elements for objects (CLOS classes) that you define in your own code. This is an application of the more general idea of Moldable development. Clouseau is moldable, SLIME's inspector is not.
Moldable development results from making customization a routine activity in software development. Developers write custom views for their objects as part of designing and implementing these objects, in order to enhance their own understanding of the code they are working on. This requires customization to be cheap, to the point that it becomes feasible to write single-use tools. You can think of a Unix shell as an early moldable tool, since it can be customized by writing shell scripts.
This inspector is customizable by adding or overriding views for specific classes or objects. In an inspector pane, such as the one you are looking at, each view occupies one tab. By clicking on a tab, you switch to a different view.
A view consists of HTML code with embedded references (to other objects) and action buttons. Learn how to define your own views.