You can select an item by clicking on it, and then open it, find the original item in the Finder, or open the Finder's Get Info window using the commands in the File menu. For folders and disks, you can open the Finder's File Sharing window using the Sharing item in the File menu.
You can also double-click the item to open it as in the Finder. From the Dock Options window for each dock described below, you can set the dock to use just a single-click to open items, so whenever this manual mentions "double-click", it could be changed to "single-click". When the single-click option is on, you can hold down Shift and click the item to just select it and not open it.
If you open an item in the Process Dock, DragThing will switch that application to the front, assuming it's not a background-only application.
You can select more than one item at a time just like in the Finder, by holding down the Shift key and clicking more items, or by clicking in an empty place on a dock and dragging a rectangle around the items you want selected.
Anything you can do to one item such as opening or deleting it, you can do to many items at once.