Addons are CMSimple_XH improvements, enhancements or simply changes which are added to the CMS in a way other than as plugins. Addons can be a bit of code which is inserted into the CMSimple code; they can also consist of some extra files which are included via template.
Using addons might complicate future updates of CMSimple_XH (see customization).
Several addons are listed in the CMSimple wiki. Please note that these addons do not necessarily fullfil the CMSimple_XH quality standards, and that they even might not be compatible with CMSimple_XH at all.