Praise be to Allah.
Meaning of fitrah
Al-Tabari (may Allah have mercy on him) said in his Tafsir: Fitrah : the deen (way or religion) of Allah. Al-Tabaris tafsir (commentary) of the ayah (interpretation of the meaning): [Iblis said] and indeed I will order them to change the nature created by Allah. [al-Nisa 4:119]
Concerning the ayah (interpretation of the meaning), “So set your face steadily and truly to the Faith: (establish) Allah’s handiwork according to the pattern (fitrah) on which He has made mankind: no change (let there be) in the work (wrought) by Allah: that is the standard Religion: but most among mankind understand not. [al-Rum 30:30 Yusuf Ali’s translation], it was reported from some of the scholars of tafsir that the phrase the pattern (fitrah) on which He has made mankind means the design of Allah according to which He has created mankind.
What are Sunan al-Fitrah ?
This word (fitrah) was also mentioned in the hadith narrated by Abu Hurayrah who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: Five things are part of the fitrah: removing the pubic hair, circumcision , trimming the moustache, plucking the armpit hairs, and trimming the nails. (Reported by al-jama'ah)
What is meant by these five things being part of the fitrah is that when they are done, this is in accordance with the natural pattern on which Allah made mankind and urged them to follow, so that they will be better and more perfect. This is the ancient sunnah (way) which was followed by all the prophets and which was enjoined by all the laws they brought. It is a natural and innate way. (Al-Shawkani, Nayl al-Awtar, Bab Sunan al-Fitrah)
And Allah knows best.