La ilaha il Allah means: There is no god but God ( #Allah in Arabic). During the pagan times, the Arabs knew who God (Allah) was as the real God and they used to pray to Him for forgiveness but they also had many other false gods and idols that they prayed to and made sacrifices for. This phrase is the first part of #shahada ( testimony of faith) To believe in Allah you must reject all other "gods" and fake beliefs.
There are numerous virtues of reciting La ilaha illallah. Among them are the following: ➡Abu Hurayrah (radiyallahu ‘anhu) once asked the #prophet (sallallahu ‘alayhi wa sallam), “Who will be the fortunate person to gain your intercession on the day of Qiyamah?” prophet Muhammad (sallallahu ‘alayhi wa sallam) said: “O Abu Hurayrah, I knew you would be the first to ask me about this due to your eagerness for Hadith. The fortunate person who will gain my intercession will be that person who says ‘La ilaha illallah’ sincerely from his heart.” (Sahih Bukhari, Hadith: 99) ➡“When a person sincerely says ‘La ilaha illallah’, the doors of the sky are opened for it until it reaches the Throne so long as he avoids major sins” (Sunan Tirmidhi, Hadith: 3590. Graded sound -hasan- by Imam Tirmidhi) ➡“The best dhikr is ‘La ilaha illallah, and the best du’a is ‘Alhamdulillah” (Sunan Tirmidhi, Hadith: 3383, Sunan Ibn Majah, Hadith: 3800, Sahih Ibn Hibban; Al Ihsan, Hadith: 843 and Mustadrak Hakim, vol. 1 pg. 498)