Code Editor is an area within Site Admin, for developers to input and edit code. Within Code Editor, you can edit "Assets" and "Layouts" for this Site.
"Assets" include CSS and JS files, whereas "Layouts" include the default and custom layouts for all of your Site's Categories, Forms, Modules, and WebApps.
Within Code Editor, you can make the editor full screen; use quick shortcuts from the top right-hand side; and compare custom layouts with default layouts (locked layouts that are automatically updated when you modify the structure of Categories, Forms, Modules or WebApps).
Where can you find Code Editor?
While viewing your Site Admin, use the left-hand menu to click "Site Manager" and expand the sub-menu. Next click on "Code Editor" to view "Assets" and "Layouts" on your Site.