Selective Sync is a feature of the Dropbox desktop application that allows you to select only the folders you want to be synced to your computer. Selective Sync gives you the control to ensure you have only the files you need on any computer. It's especially handy if you want to save space on netbooks and other computers with small hard drives.
Select which folders to sync on Mac OS X
1. Click on the Dropbox icon from the menu bar.
2. Click on the gear icon and select Preferences... from the menu.
3. Click the Account tab. (Dropbox Business users: If you have connected your personal and work Dropboxes, you'll also need to select the Dropbox for which you want to change settings at the top of the tab.)
4. Click the Change Settings... button.
The selective sync options from Dropbox preferences
A window will appear with a list of all the top level folders in your Dropbox folder. The folders with a check next to them will be synced to your computer. Uncheck any folders that you don't need to sync to your computer's hard drive. Any folders you deselected will be removed from your hard drive, but will still be available through the website and on any computers linked to your Dropbox account.
The above is an excerpt from the full support article provided by Dropbox Help Center, found here.