The Krishna paksha Ashtami that occurs after Paurnami tithi during Maagha maasam, is Ashtaka. The next day is Anvashtaka, which is because it follows Ashtami, it is called Anvashtaka.
Maagha maasam is pitru maasam, suitable for departed ancestors. The Pitru-s are the devataa for the Magha nakshatram. When Paurnami occurs in that nakshatram, it is called Maagha maasam. According to our calculations, it is Masi Maasam. As the Ashtami that occurs in this Pitru maasam is a Pitru tithi, it is special. It is said in the Smriti-s that one must perform Ekashtaka sraddha. Since it is special for Pitru-s we perform the tarpanam as a result.
‘Anu’ means that which follows. It is mentioned in the scriptures that Anvashtaka tarpanam must be performed the day after Ashtami.