This verse summarizes all the avataras since some like Hayagriva and Hamsa were not mentioned in this list. A comparison is made with a treasure house (sattva-nidheh) or an ocean of pure suddha-sattva forms of eternity, knowledge and bliss, because the forms cannot be counted. Avidasinah means inexhaustible. Normally by giving out water, a lake becomes exhausted. Countless avataras arise from the Lord like thousands of small streams (kulyah), natural torrents, flowing from an inexhaustible lake. Asankhyeya means not countable. However there is another meaning: not well known (asamyak khyata). Some like the purusavataras are well known. Others are not so well known. Prahlada has said:

ittham nr-tiryag-rsi-deva-jhasavatarair
lokan vibhavayasi hamsi jagat pratipan |
dharmam maha-purusa pasi yuganuvrttam
channah kalau yad abhavas tri-yugo ’tha sa tvam ||

In this way, by appearing in various incarnations as a human being, an animal, a great saint, a demigod, a fish or a tortoise, you maintain the worlds, kill those who cause disturbance and protect dharma in all the yugas. Great Lord! Since you will appear covered in Kali-yuga, you are called Tri-yuga. SB 7.9.38

Channah indicates that the avatara is not well known.