Essentially, the CLA states that you grant Alkacon Software the unrestricted right to use your contributions in the OpenCms distribution. At the same time, you do not guarantee that your contributions will actually work.
We regret that these legal precautions are necessary, but we believe that it is better for all OpenCms users if legal issues are clarified from the start.
Note that the OpenCms CLA is derived from the "Corporate Contributor Licence Agreement" used by the Apache Software Foundation (see http://www.apache.org/licenses/). This means that the terms of the CLA are generally recognised in the open source community and presumably a lot of work has been done by lawyers to make them as safe as possible for all parties.
If you have any questions about the CLA, please contact us.