This is one of numerous “sati” compounds of the same kind: Dhammānussati, saṅghānussati, sīlānussati, cāgānussati,devatānussati, ānāpānassati,maraṇassati, kāyagatāsati, upasamānussati…
In all of them, “sati” means “keeping in mind” that which is indicated by the first part of the compound. As explained in the Satipaṭṭhānavibhaṅga:
“Satimā” ti. Tattha, katamā sati?
“Mindful”. Herein, what is mindfulness?
Yā sati anussati paṭissati sati saraṇatā,
That which is mindfulness, recollection, recall, mindfulness, remembrance,
dhāraṇatā apilāpanatā asammussanatā,
bearing (in mind), not losing, not confusing,
sati Satindriyaṁ Satibalaṁ Sammāsati – ayaṁ vuccati “sati”.
mindfulness, the Faculty of Mindfulness, the Strength of Mindfulness, Right Mindfulness – this is called “mindfulness”.
Thus Dhammānussati means “Keeping the Dhamma in mind”, or “retention of the Dhamma”. Similarly with ānāpānassati.