The meaning of the second Shahadah, which is ‘Ashhadu anna Muhammadar-Rasulullah is: I declare with my tongue and I believe, and accept in my heart that Muhammad, salla-l-lahu ^alayhi wa sallam, was sent to all the humans and  jinn. He is truthful in everything he conveys from Allah, so that they believe in the rules he brought and follow him. Allah ta^ala said in Surat Al-Furqan, verse #1:

﴿تبارك الذي نزّل الفرقان على عبده ليكون للعالمين نذيراً﴾ سورة الفرقان.

The verse means: […due glorification is to Allah, the One Who revealed the Qur’an to His slave, to warn the humans and the jinn.]