A pick whip is a UI widget that looks like a coil on a button. When you want to link content for example placing an image on the page, you can use it. Let's say you can see your Finder window beside the InD document window. To place an image, you'd select the image box, then drag from the pick whip widget (presumably on the options or control bar). It drags out a little line and then you put the end of the line on the image file in the Finder window. And when you release it, it does a fun little wiggly noodle retraction. And the content is linked or placed.
The downside of that is it assumes that you can see your other content/files beside your document window, or you are using a file viewer inside of the application and have it displaying the content you want to link to.