The link cursor provides an alternative method for creating links between elements using the browser and input/output interfaces rather than the element's editing dialog.
In particular, double-clicking on an input or an output object in the browser or interface invokes the link cursor. Moving to a compatible (in terms of type, order and dimensions) input or output (in the browser or input/output interface) and double-clicking on it creates a link. If the link is compatible, both "boxes" on the cursor will turn green when you place the link cursor over the input/output to which you wish to link (initially, the bottom box is green and the top box is red)
You can deactivate the link cursor (returning it to a normal cursor) by right-clicking anywhere in the screen or by pressing the Esc key.