It's a reference to "la interna ideo", which Zamenhof described like this: "Lala interna ideo of Esperanto, which isZamenhof described 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, quickly translated here by me) 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)