أَيْ تَصْدِيقُ النَّبِيِّ ﷺ بِالْقَلْبِ
That is, inwardly affirming the Prophet ﷺ.
This entails accepting as true what he decisively conveyed from Allah as religion. It does not license attaching unverified historical reports to the message and treating them as necessary articles of faith.
Example: Affirming a firmly established teaching while refusing to make an unverified story into a creed both reflect care for the content of revelation. Proving attribution and assenting to its content must be kept distinct.