Bitmap Gallery - Fill Button
The Fill button in the Bitmap Gallery applies the bitmap selected in the gallery as a bitmap fill to the selected objects. If no objects are currently selected, the current fill attributes are set to use this bitmap as a fill. This means all objects drawn from then on will be filled with the bitmap.
You can also drag-and-drop the bitmap onto an object to apply it as a bitmap fill (CTRL-drag to apply inside) , double-click it to apply it to the selection (CTRL-double-clicking closes the gallery afterwards) or right-click on the bitmap in the gallery to open the pop-up menu and choose Apply as a Fill.
For details of editing the bitmap fill afterwards, see Editing a bitmap fill.