What does Islam mean?

'Islam' is an Arabic word that means 'Submission', the submission to the will of Allah/God.

How does this submission work?
If you are certain (belief is not enough, you need to be totally convinced) that there's a single creator for this universe and Mohammad was the last messenger of this creator, then you unconditionally submit your will to Allah and follow the guidance and instructions of His message which was delivered by His messenger Mohammad.

You become a Muslim by declaring Al-Shahadah
as follows:
"ASH-HADU ANLA ELAHA ILLA-ALLAH WA ASH-HADU ANNA MOHAMMADAN RASUL-ALLAH".

The English translation is:

"I bear witness that there is no deity (none truely to be worshipped) but, Allah, and I bear witness that Mohammad is the messenger of Allah".

How does this submission effectively work?
By giving yourself up to the will of Allah. There is five types of instructions in the message of Allah:
1. Haram (forbidden/prohibited): Things that you will be rewarded for if you don't do and punished for if you do, such as: to lie, to steal, to squander, drinking alcohol, drugs, eating pork, ...
2. Wajib (mandatory): Things that you will be rewarded for if you do and be punished for if you don't, such as: five prayers a day, fasting Ramadan, Hajj/pilgrimage, ...
3. Mustahab (diserable): Things that you will be rewarded for if you do and won't be punished for if you don't, such as: donation, to be clean, helping people, ...
4. Makrooh (undiserable): Things that you will be rewarded for if you don't do and won't be punished for if you do, such as: to be stingy, rude behavior, avoiding helping people when you can, ...
5. Mubah/Halal (permissible): Anything that doesn't fit into the four categories above, you don't get punished or rewarded for doing.

For instance, you may love drinking alcohol, you won't drink it because Allah made it forbidden/Haram, so you basically gave up what you like for the will of Allah.