Many different explanations have been given regarding to the two terms shaf' and watr, as they have been used in the holy Quran (surah al-fajr, verse 3).
Al-Shaf' is usually translated as the 'even number', and al-watr is translated as odd number.
One meaning given is that all creation is made of pairs, and Almighty God being Singular.
Another meaning is the Shaf' prayer being a even number of units (i.e. two rak'ahs), and the Watr prayer being an odd number of units (i.e. one rak'ah).
May the Almighty grant us the tawfeeq of upholding such a spiritual and important practice of praying Salat al-Layl.