السَّلَامُ عَلَيْكُمْ

Ittibaa’ means to follow, and is not usually used in a literal sense (i.e., following someone around), but in the sense of following a religion or way, or following your desires.

Just like laa ilaaha il-Allah is the statement of tawheed, Muhammadun rasool-ullah is the statement of ittibaa’. We know the English meaning is simple: Muhammad is the Messenger of Allah; however, this statement really means three things:

1. That you believe in what he told you – especially his prophethood and those things that relate to the unseen, like what he told you about Allah, the angels, the jinn, the Day of Judgment, Paradise, the Hellfire, prophecies of what is to come.
2. That you obey him in what he commanded – whether that is something he commanded you to do, or forbade you from doing, or something he recommended to do or not to do. The recommended things are not obligatory, but they increase you in imaan and they fulfil the true meaning of Muhammadun Rasool-ullah.
3. That you only worship Allah in the way that he showed you.

Allah knows best.

Sources:

Lecture Notes:
https://www.digitalmimbar.com/AqeedahNotesTimHumble.pdf