Shahada is a very important belief in Islam. It is the declaration that there is one god, Allah, and that Muhammad is his prophet.
Basically, Muslims believe in only one god, and that Muhammad was the person chosen by God to share his message with the world. When a person recites the Shahada, they are saying that they believe in this belief and will follow the teachings of Allah and Muhammad.
It is considered the most fundamental aspect of Islam and the first of the Five Pillars of Islam. People repeat the Shahada regularly as a way of affirming their faith in Allah and acknowledging their commitment to following Islam.
So, the Shahada is like a promise that a person makes to Allah that they believe in him and they will live their life in a way that follows his teachings.