Matsya is the first avatar of Vishnu, appearing during a great deluge. The deity Matsya derives his name from the word Matsya (Sanskrit: मत्स्य), meaning "fish".
Matsya is a hybrid form—half-fish and half-man. Fish are symbolic of life, adaptation, and survival in water, which in Hindu cosmology represents the oceanic abyss or the primordial waters that existed before the creation of the world.
King Manu, the first human or progenitor of humanity, was performing penance by the river when he encountered a small fish that requested his protection. Manu placed the fish in a jar, but as time passed, the fish grew too large for the jar, then too large for a tank, and finally too large for a lake. Realising the fish was divine, Manu placed it in the ocean. Matsya instructed Manu to build a large boat and take with him the Sapta Rishi (the seven great sages), seeds of all plants, and pairs of all animals to ensure the preservation of life.
It is often compared with the Genesis narrative of the flood and Noah's Ark. In the Mahabharata and the Puranas, the flood myth is in fact a cosmogonic myth. The deluge symbolizes dissolution of universe (pralaya); while Matsya "allegorizes" the Creator-god (Brahma or Vishnu), who recreates the universe after the great destruction. This link to Creation may be associated with Matsya regarded as Vishnu's first avatar.
Matsya, in the later versions, slays a demon named Hayagriva who steals the Vedas, and thus is lauded as the saviour of the scriptures
Matsya Stotram - https://youtu.be/BX1NXCXYZhA?si=4Z1Fo-jlHT_WHu7o
नूनं त्वं भगवान् साक्षाद्धरिर्नारायणोऽव्ययः ।
अनुग्रहायभूतानां धत्से रूपं जलौकसाम् ॥
नमस्ते पुरुषश्रेष्ठ स्थित्युत्पत्यप्ययेश्वर ।
भक्तानां नः प्रपन्नानां मुख्यो ह्यात्मगतिर्विभो ॥
सर्वे लीलावतारास्ते भूतानां भूतिहेतवः ।
ज्ञातुमिच्छाम्यदो रूपं यदर्थं भवता धृतम्
न तेऽरविन्दाक्षपदोपसर्पणं
मृषा भावेत्सर्व सुहृत्प्रियात्मनः ।
यथेतरेषां पृथगात्मनां सतां
-मदीदृशो यद्वपुरद्भुतं हि नः
इति श्रीमद्भागवते चतुर्विंशतितमाध्याये मत्स्य स्तोत्रम् ॥