Q 18: What does the testimony, "There is no god but Allah" mean?
A ---It means that none has the right to be worshipped but Allah Who has no partners or rivals with Him in Worship or Dominion. Allah the Most High says:
That is because Allah -He is the Truth (the only True God of all that exists, Who has no partners or rivals with Him), and what they (the polytheists) invoke besides Him, it is Balil (falsehood). And verily, Allah - He is the Most High, the Most Great. (Al-Hajj, 62)
Reference: 200 FAQ On Muslims Belief - Shiekh Hafiz Ibn Ahmed Al Hakami