What does the shahada mean?

The words

In Arabic: Ash-hadu an la ilaha illa Allah, wa ash-hadu anna Muhammadan rasul Allah.

• Ash-hadu: "I bear witness" or "I testify". It is a sincere declaration, like giving evidence.
• La ilaha: "there is no god", meaning nothing deserves worship.
• Illa Allah: "except Allah", the One God who created everything. Allah is simply the Arabic word for God; Arabic-speaking Christians and Jews use it too.
• Rasul Allah: "Messenger of Allah", someone chosen to deliver God's message.

Part one: "There is no god but Allah"

This sentence first rejects and then affirms. It rejects worshipping anything else: idols, people, money, nature or your own desires. Then it affirms worship of Allah alone. It is the heart of Islam's message of tawhid (the oneness of God), and the Quran says it was the message of every prophet:

So you are not turning away from the God of Abraham, Moses or Jesus, but affirming that He is one and that nothing is worshipped beside Him. See What is tawhid in Islam?

Part two: "Muhammad is the Messenger of Allah"

Muslims believe Muhammad ﷺ was a human being, not divine, chosen to deliver the Quran. He is not worshipped; he is followed. Accepting him as Allah's messenger means:

• believing what he taught about God, the afterlife and how to live;
• loving and respecting him, while remembering that he is a servant of Allah;
• following his example as best you can, learning step by step.

The Quran also calls him the last of the prophets:

To learn what makes the declaration sincere, see What are the conditions of the shahada?

What it does not require

The shahada doesn't demand perfect Arabic, full knowledge or a perfect life. It asks for honest belief and a willingness to learn and practise. The rest of Islam, such as prayer, fasting and charity, grows from this one declaration.