Your data on the drive is very important and is at risk of being tainted. Just deleting the data will not completely erase the data present in the files. Deletion just marks a file as deleted, while the contents of the file still exist on the hard drive. It is therefore, a general standard to erase and rewrite over a drive at least three times, to ensure complete deletion of data.
In order to erase free disk space, click on 'Erase Free Disk Space' button at the toolbar or click on 'Erase' further, click on 'Erase Free Disk Space' in menu bar. As you click the button, a window to select drives is displayed and you need to select the drive/partition to be erased and finally select the erasing/wiping standards to erase the free disk space.