It's a reference to la interna ideo, which Zamenhof described in this way.
La interna ideo of Esperanto, which is in no way obligatory for any individual Esperantist, but which [...] is and always should be key during Esperanto Congresses, is this: with a neutral linguistic foundation, that barriers between peoples be removed and that people become accustomed to seeing others simply as people and as brothers.
(L. L. Zamenhof, 1912, translated by me)
