Both FileZilla and FileZilla Server are free open-source software distributed under the Terms and Conditions of the GNU General Public License (GPL) version 2 or (at your option) any later version.
For using FileZilla and FileZilla Server, no restrictions apply. You can further redistribute and/or modify this software under the terms of the GPL.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
For details, have a look at the full text of the GPL.