Answer: Expressing the unity of Allah.
In Arabic, the index or fore finger is called Musabbiha (مُسَبِّحة), mostly used with the definite article: al-Musabbiha (الْمُسَبِّحة). Sometimes also this word is used: al-Sabbāha (السَّبّاحة).
This finger, when a Muslim makes a sign with it (by raising it), symbolises the unity of Allah.
Let’s dig deeper. You might know the Arabic word Subhāna (سُبْحانَ). The expression “Subhāna Allah” (سُبْحانَ الله) expresses a Muslim’s admiration as an exclamation of surprise. It is used a lot in the daily life of Muslims. We could translate it as: Praise Allah! Or: Allah be praised!
Let’s have a look at another Arabic word derived from the same root: A rosary is called Misbaha (مِسْبَحة) or Subha (سُبْحة). It consists of beads on a string, 99 in number, and having a mark after each 33 – with which a Muslim performs the act called al-Tasbīh (التَّسْبِيح) meaning the repetition of the praises of Allah. Usually Muslims repeat the expressions Subhana Allah and Allāhu ’akbar.