Dropbox stores every file you delete for 30 days, giving you the opportunity to restore it whenever you need. To do this, go to dropbox.com and navigate to the folder where the file (or folder) that you deleted was. Then click the Show deleted files icon.
All your deleted files and folders will now be displayed, and you can right click on them to restore them.
You can also delete folders and files permanently to give you more space in your Dropbox.