The word Fitrah has various meanings. In the Hadith you mention, Fitrah means such a natural disposition in which is vested the quality of accepting the trust. Thus, if the child had to grow up with no outside influence whatsoever, naturally, the person would turn towards the worship of one Allah. Qur'an can be found meaning al-insyiqaq or al-shaqq which means glassware. In addition, fitrah also means al-khalqah al ijad or al-ibda' which means creation. That meaning means that every human being is free from stains, sins, as can be understood from the following verse: يْنَِب ْۢنْمِ َ كُّبرَ َ ذ َ خ َ ا ْ RZswttT.