The so-called Stack allows you to get an impression of a piece of content in the system while remaining in the current context. This is especially handy in Explorer when using the Radar or Portfolio Visualizations.
Expand/Collapse the Stack
Click the button (window resize icon) on the upper left part of the stack card in front to expand or collapse the Stack.
Open detail page in new tab
Click the button (diagonal arrow icon) on the upper left part of the stack card in front to open the detail page of an element in a separate browser tab.
Close the Stack
Click the "X" button on the upper right part of the stack card in front to close it. Hovering the button with more than one card on the stack opens an additional button to "Close All".
Minimize the Stack
Click on the colored upper bar (collapsed view) or sidebar (expanded view) of the stack. This allows you to navigate the Explorer on smaller screens without losing the history of the stack.