In the Name of Allah, With Whose Name Nothing Can Harm
بِسْمِ اللَّهِ الَّذِي لَا يَضُرُّ مَعَ اسْمِهِ شَيْءٌ فِي الْأَرْضِ وَلَا فِي السَّمَاءِ وَهُوَ السَّمِيعُ الْعَلِيمُ
Bismillahil-ladhi la yadurru ma'asmihi shay'un fil-ardi wa la fis-sama'i wa huwas-sami'ul-'alim.
In the name of Allah, with whose name nothing in the earth or in the heaven can cause harm, and He is the All-Hearing, the All-Knowing.
What it means
A short sentence with a large claim: nothing anywhere can harm a person whom Allah has shielded. It is one of the easiest of the daily adhkar to memorise, which makes it a good first one to teach children.
When and how
Say it three times in the morning and three times in the evening.
Its virtue
The Prophet ﷺ said that whoever says it three times in the morning and three times in the evening will not be struck by any sudden affliction, reported in Sunan Abi Dawud 5088 and Jami at-Tirmidhi 3388.
Source
Abu Dawud 5088; At-Tirmidhi 3388sahih