Shahada literally means to bear witness and in this case it means that a Muslim attests and bears witness to two facts: 1) There is only one God, Allah (SWT), and 2) Mohammed (SAW) is Allah's Messenger.

The wording of Shahada goes as follows: Ash'Hadoo Allaa Ilaaha Il'La Allah wa Ash'Hadoo An'na Muhammadan Rasooloo Allah = I bear witness that there is no god but Allah and that Mohammed is a Messenger of Allah.