This calligraphy in Kufic style of writing is inspired on the 39th verse of Surah Al Kahf in which one of the two neighbours, whose life showed more signs of "abundance" and had the nicest garden between the two, finds one day his garden destroyed because he did not show appreciation for that what Allah (swt) had provided him. When his neighbour finds out what has happened to his garden, he asks:
وَلَوْلَا إِذْ دَخَلْتَ جَنَّتَكَ قُلْتَ مَا شَاءَ اللَّهُ لَا قُوَّةَ إِلَّا بِاللَّهِ ۚ إِن تَرَنِ أَنَا أَقَلَّ مِنكَ مَالًا وَوَلَدًا -
And why did you, when you entered your garden, not say, 'What Allah willed [has occurred]; there is no power except in Allah '? Although you see me less than you in wealth and children, 18:39
The door that is illustrated through this calligraphy is also a symbolic reminder to us all to recite "..Bismillah, Mashallah, la quwwata illa billah.." that could be translated as "..In the name of Allah, (only) Allah’s Will will be done, there is no power but with Allah..” as a tasbih recitation or meditational prayer for protection when even we go in or out of any place and so pass through a door.
NB: No translation of the Holy Quran can reflect the full debts of its original revelation in Arab and always has to be considered as one of sometimes multiple meanings or ways of translating that is possible when translated in any language.