What does "Bismillah" mean?

The short and the full version

"Bismillah" is short for "Bismillahir-Rahmanir-Rahim": "In the name of Allah, the Entirely Merciful, the Especially Merciful." This full phrase, called the basmala, opens the Quran and appears at the start of every surah except one (Surah at-Tawbah).

Notice which names of Allah are chosen: both come from rahmah, mercy. Every time you start something with Bismillah, you begin by reminding yourself that Allah is merciful.

What it means when you say it

Saying "Bismillah" carries a hidden verb: "In the name of Allah I begin", "I eat", "I go out". You're saying that you do this action with Allah's permission, relying on His help, and hoping it pleases Him. It's a tiny phrase that turns an ordinary moment into worship.

The Quran also shows it at the top of a letter: the Prophet Sulayman's letter to the Queen of Sheba began with "Bismillahir-Rahmanir-Rahim" (Quran 27:30).

When do Muslims say it?

• Before eating and drinking. The Prophet ﷺ told a boy: "Say Bismillah, eat with your right hand, and eat from what is in front of you" (Sahih al-Bukhari 5376). See What dua do I say before eating?
• Entering the home. When a person mentions Allah on entering and at meals, Shaytan says to his companions, "You have no place to stay and no dinner" (Sahih Muslim 2018).
• Leaving the house, wudu, getting dressed, travelling and starting any good task. See What dua do I say when leaving the house?
• Before reciting a surah of the Quran.

Muslims don't say it before something haram, since you can't ask Allah's blessing on a sin.

What if I forget?

Don't worry, it's very common for new Muslims. If you remember halfway through a meal, you can say "Bismillahi awwalahu wa akhirahu" (In the name of Allah, at its beginning and its end). See What do I say if I forget to say bismillah before eating?