A hack is a piece of code that modifies the official code of the XOOPS core or a specific module. Hacks are generally adding new functionnalities or different behavior to an application. Generally, if a hack is usefull and fits with the general roadmap of the developpement team, it will later be implemented in the application.

When adding hacks to your site, you need to make sure to document them as it will be harder to upgrade the application when a new vesion comes in. Hacks and upgrade can be in conflicts. The only way to solve this is to merge the files with a Files Merging tool like BeyondCompare or WinMerge.

Althought hacks can be really usefull for specific purposes, you will need some basic PHP and HTML knowledge to implement them correctly and also, to be able to upgrade the application afterwards. It is not recommended for new users.