What does saying Muhammadan rasulullah means?

 What does saying Muhammadan rasulullah means?

Alhamdulillah, a person can never be Muslim if he does not utter the two creed in Islam which is the kalimatul shahadayn saying 

اشهد ان لا اله الا الله واشهد ان محمدا عبده ورسوله.

One is to proclaim this with his tongue with the heart is certain of it and actions confirming it.

Avoiding going to soothsayers, saying amulets and charm for protections, worshipping prophets, saints and tombs.

Saying Muhammad is his messenger is the second creed in Islam.

This kalimatul shahadayn can be found in Quran.

Allah says,

Aal-e-Imran 3:18

شَهِدَ ٱللَّهُ أَنَّهُۥ لَآ إِلَٰهَ إِلَّا هُوَ وَٱلْمَلَٰٓئِكَةُ وَأُو۟لُوا۟ ٱلْعِلْمِ قَآئِمًۢا بِٱلْقِسْطِۚ لَآ إِلَٰهَ إِلَّا هُوَ ٱلْعَزِيزُ ٱلْحَكِيمُ 

Allāh witnesses that there is no deity except Him, and [so do] the angels and those of knowledge - [that He is] maintaining [creation] in justice. There is no deity except Him, the Exalted in Might, the Wise.

Allah himself confirms he is one and alone same as the angels and people having knowledge of the scriptures.

The second creed,

Allah says,

Al-Fath 48:29

مُّحَمَّدٌ رَّسُولُ ٱللَّهِۚ وَٱلَّذِينَ مَعَهُۥٓ أَشِدَّآءُ عَلَى ٱلْكُفَّارِ رُحَمَآءُ بَيْنَهُمْۖ تَرَىٰهُمْ رُكَّعًا سُجَّدًا يَبْتَغُونَ فَضْلًا مِّنَ ٱللَّهِ وَرِضْوَٰنًاۖ سِيمَاهُمْ فِى وُجُوهِهِم مِّنْ أَثَرِ ٱلسُّجُودِۚ ذَٰلِكَ مَثَلُهُمْ فِى ٱلتَّوْرَىٰةِۚ وَمَثَلُهُمْ فِى ٱلْإِنجِيلِ كَزَرْعٍ أَخْرَجَ شَطْـَٔهُۥ فَـَٔازَرَهُۥ فَٱسْتَغْلَظَ فَٱسْتَوَىٰ عَلَىٰ سُوقِهِۦ يُعْجِبُ ٱلزُّرَّاعَ لِيَغِيظَ بِهِمُ ٱلْكُفَّارَۗ وَعَدَ ٱللَّهُ ٱلَّذِينَ ءَامَنُوا۟ وَعَمِلُوا۟ ٱلصَّٰلِحَٰتِ مِنْهُم مَّغْفِرَةً وَأَجْرًا عَظِيمًۢا 

English - Sahih International

Muḥammad is the Messenger of Allāh; and those with him are forceful against the disbelievers, merciful among themselves. You see them bowing and prostrating [in prayer], seeking bounty from Allāh and [His] pleasure. Their sign is in their faces from the effect of prostration [i.e., prayer]. That is their description in the Torah. And their description in the Gospel is as a plant which produces its offshoots and strengthens them so they grow firm and stand upon their stalks, delighting the sowers - so that He [i.e., Allāh] may enrage by them[1] the disbelievers. Allāh has promised those who believe and do righteous deeds among them forgiveness and a great reward.

Muhammad is the messenger of ALLAH and believing in this is to abide by the examples and his teachings,practicing what he orders us and refraining what what he prohibits us.