The musnad ilayh is the element about which something is predicated. It can appear in several grammatical forms, including the subject of an active verb, the subject of a passive verb, and a mubtadaʾ with a predicate.
“جَاءَ زَيْدٌ”
“Zayd came.”
“زَيْدٌ” is the grammatical subject and musnad ilayh: arrival is attributed to him.
“كُتِبَ الدَّرْسُ”
“The lesson was written.”
“الدَّرْسُ” is the passive subject and musnad ilayh.
“زَيْدٌ حَاضِرٌ”
“Zayd is present.”
“زَيْدٌ” is the mubtadaʾ and musnad ilayh; “حَاضِرٌ” is its predicate.