This was regarding the definition. Let’s see some hadith:

The Prophet (peace be upon him) said, “Islam is to testify there is no God but Allah and Muhammad is the Messenger of Allah, to establish prayer, to give charity, to fast the month of Ramadan, and to perform pilgrimage to the House if a way is possible.”
Source: Ṣaḥīḥ Muslim 8
Grade: Sahih (authentic) according to Muslim

Narrated by Anas bin Malik
The Prophet (peace be upon him) said: “Islam is to declare, but the faith is in the heart”, then he rested his hand on his chest and said: “The piety (takwa) is here, the piety is here.”

Has it been observed? In the second hadith, the Prophet (peace be upon him) considers Shahada to be an entrant to Islam, but only as a declaration that is manifested in words. Even a parrot can repeat the words of the testimony. If the words are not saturated with content, i.e. faith that dwells in the heart, those are certainly not worth much. The difference between the declaration of Islam and the actual faith appears also in the Quran, which is a much stronger justification of the same thought:

The desert Arabs say, "We believe." Say, "Ye have no faith; but ye (only)say, 'We have submitted our wills to Allah,' For not yet has Faith entered your hearts. But if ye obey Allah and His Messenger, He will not belittle aught of your deeds: for Allah is Oft-Forgiving, Most Merciful." (Quran 49:14)

The desert Arabs were somewhat shaky in their faith. Their hearts and minds were petty, and they thought of petty things, while Islam requires the complete submission of one's being to Allah. Some of the failings of the desert Arabs are described in 48:11-15. But the reference here is said to be to the Banu Asad, who came to profess Islam in order to get charity during a famine.

Those are in the wrong track who think that they open the heart of somebody towards Islam by making newcomers to pronounce the words of Shahadah. They are at a level of knowledge that alienates converts from Islam in the long run. We must not confuse the heat of Dawah (calling to Islam) with knowledge. Unfortunate converts, they, too, are astray when think to begin a whole new life after saying the words of the Shahada, denying their old faith, while it was another monotheistic religion. No. They do not deny anything. They just see it differently. They receive a different kind of knowledge, but they do not exchange faith or God. They change religion, not faith. Why not? That's why:

O ye who believe! Believe in Allah and His Messenger, and the scripture which He hath sent to His Messenger and the scripture which He sent to those before (him). Any who denieth Allah, His angels, His Books, His Messengers, and the Day of Judgment, hath gone far, far astray. (Quran 4:136)

This is the clear Statement of the Monotheism. No matter which Book or Prophet, or Angel if one has true belief in them and in Allah and in the Day of Judgement, he has to be considered believers.

Now, let's put the thought together.
The one who utters the words of testimony in Islam will not adopt the religion of the Arabs, will not be follower of an Arab guy named Mohamed, and will not have to worship the God of the Arabs. This is the misconception inside and outside Islam! Because Arabs do not have a specific religion, a specific prophet, and a specific God. Islam is the universal faith of mankind, confirming previous monotheistic religions, prophets, books, etc. and regards all of that within its own framework. Since there is a lot of prejudice against Islam, it is perhaps more understandable to call it Monotheism. It is not religion, but faith in the One God. So, all the prophets before Muhammad were also Muslims!
Yes, even Moses and Jesus! They were both Muslims. Or was it something else and not the Monotheism what they propagated? This is not me who is saying that, but the Quran. To confirm that, I recommend two of my articles titled: “Regard what is linking us together” and “Can a Muslim woman have a non-Muslim husband”. Muhammad is not an Arab guy, but the last Prophet (peace be upon him) who closes the line of Muslim (i.e., submitted to the One God) prophets.
So, what does the Shahada mean?
“I bear witness that there is no other God, than Allah.” Confession of faith regarding the service of the God of Abraham, Isaac, Ishmael, Moses, Jesus …… and other Prophets. He is Allah, Who is the same as Yahweh, God, Bog, Who is mentioned by a thousand other names. “I bear witness that Mohammed (peace be upon him) is the messenger of Allah”. A confession of faith about the credibility of all the aforementioned previous prophets who preached nothing else than Muhammad (peace be upon him), who closes the line, after him there are no more prophets.
He brought the universal Truth, the word of God, the Quran, which should unite us. This is the Shahada all about.