Briefly, that means you can distribute Textpattern CMS, in part or full, in its original form or with modifications and additions, provided you distribute it under the GPLv2. Distributing any part of Textpattern, including derived works, under any other license, is not permitted.
Textpattern CMS may be used for any purpose, including commercial websites. The license explicitly states that “the act of running the Program is not restricted” by the GPLv2.
The GNU project maintains a list of frequently asked questions about the GNU Licenses. Some that are likely to be of interest to Textpattern are summarized below.
Textpattern CMS shares memory space, functions and data structures with plugins, which suggests plugins must be distributed under the GPLv2.
Integrating Textpattern CMS with other proprietary software
Linking or integrating Textpattern CMS with proprietary software is at best problematic (limited to specific types of software interfaces, such as a fork/exec), at worst a breach of the license agreements of both software applications.
The GPLv2 doesn’t require users to make their own private modifications public:
The GPL does not require you to release your modified version. You are free to make modifications and use them privately, without ever releasing them. This applies to organizations (including companies), too; an organization can make a modified version and use it internally without ever releasing it outside the organization.
Providing a copy of Textpattern CMS to someone outside your own company is considered distribution under the GPLv2. The GPLv2 explicitly allows you to sell copies of Textpattern CMS:
However, web developers or designers who distribute a copy of Textpattern CMS (original or otherwise) to their clients, must do so under the GPLv2. You’re not required to provide a copy to anyone else, but you can’t prevent your client from copy or distributing it under the GPLv2 if he/she chooses:
This applies also to any modifications or additions to Textpattern CMS, and any integrated software or libraries that link with Textpattern code:
A system incorporating a GPLv2-covered program is an extended version of that program. The GPLv2 says that any extended version of the program must be released under the GPLv2 if it is released at all.
In short: you can charge clients for providing a copy of Textpattern CMS, plugins, modifications and addons. However, you must license all of that software to them under the GPLv2.