The Dashavatara (दशावतार) is an excerpt from Srimad Bhagavatam that speaks about the ten primary Avataras of Vishnu. Dasha (दश) means ten and Avatara (अवतार) means incarnation.
In Kūrma (कूर्म) avatāra, Bhagavān incarnated as a turtle to help the Devas and Asuras with the churning of the ocean to get amṛta (a nectar that gave everlasting life to those who drank it).
Bhagavān Viṣhṇu incarnated as a huge boar - Varāha (वराह), to rescue Mother Earth from an underwater loka (लोक) called Rasātala (रसातल) by killing the Asura called Hiraṇyākṣha (हिरण्याक्ष).