Sacred Hospitality
Ponnadi sāttrudal (Tamil: pon = golden/sacred + adi = feet + sāttrudal = to welcome/receive; 'receiving the golden feet') is the act of inviting a Śrī Vaiṣṇava to one's home and receiving them with honor. The 'golden feet' refers to the guest's feet — a Śrī Vaiṣṇava's feet are considered sacred, and receiving them in one's home is a blessing.
Extension of Divine Service
The act flows from tadīya-śeṣatva: a Bhagavān-devotee is an extension of Bhagavān Himself. Welcoming and serving such a devotee in one's home is welcoming and serving Bhagavān. This is why Piḷḷai Lokācārya teaches that in Kali Yuga, bhāgavata-sēva (service to devotees) is even more meritorious than temple worship.
Ponnadiyām Sengamalam
The phrase ponnadiyām sengamalam ('golden lotus feet') refers specifically to the lotus feet of Maṇavāḷa Māmunigaḷ. It is used in the tradition's devotional vocabulary in the same way that Śrī Rāmānujam refers to Nammāzhvār's feet at Āzhvārtirunagari — as an honored focal point of devotion.