بسم الله الرحمن الرحيم
What is the Deen-ul-Fitrah?

The deen-ul-fitrah, the religion of human nature, is Islām.  
 
Ffitrah refers to the natural state, the design of Allāh according to which He has created mankind.  Allāh subhānahu wa ta'alā said in the Qur'ān:

فَأَقِمْ وَجْهَكَ لِلدِّينِ حَنِيفًا فِطْرَةَ اللَّهِ الَّتِي فَطَرَ النَّاسَ عَلَيْهَا لَا تَبْدِيلَ لِخَلْقِ اللَّهِ ذَلِكَ الدِّينُ الْقَيِّمُ وَلَكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ
Set your face to the deen in sincerity (hanîfan) which is Allāh's fitrah (the nature made by Allāh) upon which He created mankind.  
There is no changing the creation of Allāh. That is the right deen but most people know not.  (Ar-Ruum 30:30)

The above ayah describes a fitrah of primordial faith which Allāh Himself implanted in human nature.  It implies Islām's essential message of submission to the will of Allāh as taught as practised by the prophets.

Deen means religion in the widest sense of the word, embracing both the practical aspects of the acts of worship and ordinary transactions of life, and the teachings of religion; and it is a name for that whereby one serves Allah.

إِنَّ الدِّينَ عِندَ اللّهِ الإِسْلاَمُ وَمَا اخْتَلَفَ الَّذِينَ أُوْتُواْ الْكِتَابَ إِلاَّ مِن بَعْدِ مَا جَاءهُمُ الْعِلْمُ بَغْيًا بَيْنَهُمْ وَمَن يَكْفُرْ بِآيَاتِ اللّهِ فَإِنَّ اللّهِ سَرِيعُ الْحِسَابِ
Truly, the religion (deen) in the sight of Allāh is al-Islām.  Those who were given the Scripture (Jews and Christians) did not differ except, out of mutual jealousy, after knowledge had come to them. And whoever disbelieves in the Ayat (proofs, evidences, verses, signs, revelations, etc.) of Allāh, then surely, Allāh is Swift in calling to account.  (Aal-Imrān 3:19)

The shari‘āt, which the prophets were sent with, are guiding lights to the essential faith in Allāh which is created in every human being.  Furthermore, since this faith comes from Allāh, it naturally follows that only laws capable of guiding mankind back to it must also come from Allāh, hence Islām is called deen-ul-fitrah, the religion of human nature.  The laws of Islām and its teachings are in full harmony with the normal and the natural inclination of the human fitrah to believe in and submit to the Creator.  

[For further detail, see: Definition of Fitrah