Sample Bins list samples stored in folders on your computer, giving you a pallet of sounds to quickly load into LoopTracks. (Sample Bins also list shortcuts to samples.)
When the cursor displays a little wave symbol, dragging from the Sample Bin to a LoopTrack loads the sample into the LoopTrack. Otherwise, clicking on the sample loads it into the destination LoopTrack listed in the drop down below the Sample Bin.
The Destination Drop Down determines these behaviours. For more details, see Loading Samples Using the Browser Window.