The Shahadah is the first pillar of Islam. It is an open declaration to tell the community that you believe that there is nothing worthy of worship, apart from Allah and that Muhammad is His final Messenger. Normally, this is recited in Arabic first and then in the persons own language.
“Ash-hadu an La Ilaha Illallahu, wa Ash-hadu anna Muhammadur RasulAllah.”
“I bear witness that there is no God but Allah, and I bear witness that Muhammad is the Messenger of Allah.”
This lets people know that you are a Muslim and that you acknowledge that it is Allah who is the One who has given you life and all the blessings in your life. It is Muhammad who was sent to show us how to worship Allah and is the best example to mankind. He is the last and final of all the Messengers sent to us by Allah.