Prophet Muhammad ﷺ is being referred to as that lamp (Siraajan-Muneeraa).
Imam At-Tabari says in his Tafseer of Al-Ahzaab:46:
{ وَسِرَاجاً مُنِـيراً } يقول: وضياء لـخـلقه يستضيء بـالنور الذي أتـيتهم به من عند الله عبـاده
{ مُنِـيراً } يقول: ضياء ينـير لـمن استضاء بضوئه، وعمل بـما أمره. وإنـما يعنـي بذلك، أنه يهدي به من اتبعه من أمته.
[Qatadah] said (about Siraajan-Muneeraa):
And he is a light for Allah's creation, illuminating with the light which [Prophet Muhammad] has come with from Allah to His servants
And he said (about Muneeraa):
A lamp that illuminates those who seek to be enlightened by its light, and perform what they have been commanded to do.
And what is meant by that is that he guides whoever follows him from his Ummah.
And Ibn Al-Jawzi says something similar:
{ وسراجاً منيراً } أي: أنت لِمَن اتَّبعك { سراجاً } ، أي: كالسِّراج المضيء في الظلمة يُهتدى به.
It means that you (i.e. Muhammad) are a lamp for those who follow you; in other words like a lamp that acts as a guiding light in the darkness.