From Second Life Wiki
What is Multi-select?
Every modern operating system provides a way to select multiple files or folders and do something to each of them. In 1.7 you can select a number of items in your inventory and drag them into a box or into different folders. You can drag multiple pieces of clothing onto your avatar at once. You can also delete items using multi-select. You can view multiple inventory items' properties by right-clicking a multi-selected group and choosing 'Properties'. You can also 'Open' most items in the inventory, providing you with a tabbed window in which you can view textures, read/edit notecards and modify for-sale options for objects. If only clothing items are opened they will be worn by your avatar.
You can select the contents of an object and drag all the items in the object to your inventory! No more repetitive stress injuries from dragging each item out and into your inventory individually!
Hold the Shift key and click on the first item, then the last item to add everything between the two clicked items to the selection. Hold the Ctrl key and click individual items to add or remove them from the selection. Click, hold and drag one of the selected items to move the whole selection.
As of 1.7.0, there are some limitations to using Multi-select. You can not drag objects into the world using multi-select. You can not drag no-copy items and copyable items into a folder. Only the last-selected object in a multi-selected group will be attached to an avatar, even if the objects have different attachment points.