A stranger will not understand this, a fool will not understand this
A person who is not familiar with the subject will not understand at all what is being said. In the question: Only someone who has experienced the subject can understand the emotions or details associated with the matter.
("How great are your works, O Lord! How profound are your thoughts! A foolish man does not know, and a fool does not understand this" - Psalms 118:1-2, and so on: "And they did to them as the beast does, which pursueth even so, to the fullness of its belly, with the lust of beastliness. And we say all the day long, How many are your works, O Lord! All of them in wisdom you have made, and the fool He will not understand this, and he will give it to his heart when we consider his deeds, "the light of the light" (Or HaMeir at the beginning of Parashit Vai'veer)
Translation
Expression (Hebrew)
זָר לֹא יָבִין אֶת זֹאת, כְּסִיל לֹא יָבִין אֶת זֹאת
Definition (Hebrew)
אָדָם שֶׁאֵינוֹ בָּקִי בַּנּוֹשֵׂא לֹא יָבִין כְּלָל עַל מָה הַמְּדֻבָּר. בְּהַשְׁאָלָה: רַק מִי שֶׁחָוָה אֶת הַנּוֹשֵׂא יָכוֹל לְהָבִין אֶת הָרְגָשׁוֹת אוֹ הַפְּרָטִים הַקְּשׁוּרִים בַּדָּבָר.
("מַה גָּדְלוּ מַעֲשֶׂיךָ ה' מְאֹד עָמְקוּ מַחְשְׁבֹתֶיךָ. אִישׁ בַּעַר לֹא יֵדָע וּכְסִיל לֹא יָבִין אֶת זֹאת"- תְּהִלִּים צָב וְ-ז, וְכֵן:"וּמַעֲשֵׂהוּ בָּהֶם כְּמַעֲשֵׂה הַבְּהֵמָה, הָרוֹדֶפֶת גַּם כֵּן לְמַלֹּאות בִּטְנָהּ מִתַּאֲוַת הַבַּהֲמִיּוּת, וַאֲנַחְנוּ אוֹמְרִים בְּכָל יוֹם, מָה רַבּוּ מַעֲשֶׂיךָ ה' כֻּלָּם בְּחָכְמָה עָשִׂיתָ, וְהַכְּסִיל לֹא יָבִין אֶת זֹאת, וְיִתֵּן אֶל לִבּוֹ בְּהִתְבּוֹנֵנוּ עַל מַעֲשֵׂהוּ הַזָּר"- אוֹר הַמֵּאִיר בְּרֵאשִׁית פָּרָשַׁת וַיַּרְא)