Bismillah means "In the name of Allah”. The complete form is “Bismillah al-Rahman al-Rahim”, which translates to ”In the name of Allah, the Most Gracious, the Most Merciful".

When one says “Bismillah” before starting to do anything, what that means is, “I start this action accompanied by the name of Allah or seeking help through the name of Allah, seeking blessing thereby. Allah is God, the beloved and worshipped, to Whom hearts turn in love, veneration and obedience (worship). He is al-Rahman (the Most Gracious) Whose attribute is vast mercy; and al-Rahim (the Most Merciful) Who causes that mercy to reach His creation.

Ibn Jarir (may Allah have mercy on him) said:

Allah, may He be exalted and His name sanctified, taught His Prophet Muhammad ﷺ (peace be upon him) proper manners by teaching him to mention His most beautiful names before all his actions. He commanded him to mention these attributes before starting to do anything, and made what He taught him a way for all people to follow before starting anything, words to be written at the beginning of their letters and books. The apparent meaning of these words indicates exactly what is meant by them, and it does not need to be spelled out.

Muslims say “Bismillah” before eating because of the report narrated by `A’ishah (may Allah be pleased with her), that the Prophet ﷺ (peace be upon him) said:

When one of you eats some food, let him say Bismillah, and if he forgets to do so at the beginning, let him say Bismillah fi awwalihi wa akhirihi (In the name of Allah at the beginning and end).

Sunan Ibn Majah, 3264

There is something omitted in the phrase “Bismillah” when it is said before starting to do something, which may be “I begin my action in the name of Allah,” such as saying, “In the name of Allah I read”, “In the name of Allah I write”, “In the name of Allah I ride”, and so on. Or, “My starting is in the name of Allah”, “My riding is in the name of Allah”, “My reading is in the name of Allah”, and so on. It may be that blessing comes by saying the name of Allah first, and that also conveys the meaning of starting only in the name of Allah and not in the name of anyone else.

Source: islamqa.info · islamqa.info