While Periyazwar’s “Thiruppallaandu” explains the pranavakaram “OM” in detail, Andal’s “Thiruppavai” explains “Namo Narayanaya” portion of the Thirumanthiram in detail. A classic example can be cited from Pasuram no : 6 (Pullum Silambinakaan), where while waking up Her friend, Andal says,
வெள்ளை விளிசங்கின் பேரரவம் கேட்டிலயோ
veLLai viLisangin peeraravam kETtilayoo
This line on the outside means the "Big sound of Conch". The word in fact has a lot of inner meaning to it per our Poorvaacharyaas.
Here in “peraravam (big sound)”, She gives the direct and the inner meanings of the thirumanthiram “Om Namo Narayanaya”. There are a lot of meanings to Thirumanthiram and let us focus one meaning.
Aravam (Meaning of Thirumanthiram)
1) We are adiyars only to Sriman Narayana.
2) We have to perform Kainkaryams to Sriman Narayana.
Peraravam (Inner meaning of Thirumanthiram)
1) We are adiyars to Bhaagavathaas (adiyaarkku adiyaar).
2) We have to perform kainkaryam to Bhaagavathaas.
Thus the essence of “Thirumanthiram” is conveyed in one of several ways in Thiruppavai.
Thiruppavai Upanyasams by Sri. U. Ve. Velukkudi Krishnan swami : http://www.ibiblio.org/ramanuja/thiruppavai/pasuram/