A copyleft provision in a software license means:

A copyleft provision in a software license typically means that if you redistribute the software, especially if it has been modified, you must also distribute the source code to any changes you make under the same license terms as the original software. This is the key characteristic of copyleft licenses, such as those found in the GNU General Public License (GPL).

To clarify the other options:

• You must provide free copies of the software if you use it: This is not necessarily required by copyleft licenses. What is required is that if you distribute the software (whether modified or not), the recipient must also receive the same freedoms you did, including access to the source code under the same license terms.
• You give up your copyright to the software: This is not accurate. Copyleft licenses do not require copyright holders to give up their copyright. Rather, they are allowed to retain copyright but must license the software in a way that preserves certain freedoms for users and developers.
• You may not link against third party closed source software: This is not universally true for all copyleft licenses but is a concern with some strong copyleft licenses like the GPL. These licenses may have restrictions on how the software can be combined with non-free or closed-source software.
• You must provide support for your modifications: Copyleft licenses do not require you to provide support for the software you distribute, whether modified or not. The obligation is limited to sharing the source code and maintaining the license conditions.

The core principle of a copyleft license is to ensure that the freedoms to use, modify, and share the software are preserved in all versions of the software, and this is primarily enforced through the requirement to distribute modifications under the same license terms.