Bear in mind that these two are the only type of Syahadah exist, and no other addition nor subtraction nor changes are allowed.
When you see a person who wishes to convert into Islam they must utter the Syahadah, but it is not enough. Words alone is never enough if they do not carry the same intention as the word uttered. The person who utter the Syahadah must admit with his heart and knowledge that there is no God but Allah and Muhammad is the messenger of Allah.
There must be no doubt that there exist no other God except Allah, no equal and no accomplices. Allah alone rule the whole universe. Allah alone is the almighty and all powerful and merciful with no other helping Him. Allah has 99 names and all of it is the symbol of his vast and limitless capacities. And Allah is by no means similar to any of His creations and Allah see us while we see him not, until the hereafter. He is the only one deserved to be worship and obeyed.
There must be no doubt that Muhammad is the servant and messenger of Allah to mankind, he was sent as a blessing to all the realm, to bring teaching of the One True God; Allah, and whatever he says is not from his own will but rather from the command and knowledge Allah has given to him. He is the best of example and he is exempted from committing any sins be it minor or major sins. Muhammad is the revealer of the true knowledge of Islam and we are supposed to follow his teachings as it is confirmed to save us in the hereafter.
That is the meaning of the Syahadah, to testify that there is no God but Allah and Muhammad is the messenger of Allah, by speech, by heart, and by knowledge without a single trace of doubt. And that is the first pillar of Islam.