Constellations is a character progression system in Genshin Impact. Constellations upgrade the playable character's talents/skills by altering its effects or by further increasing its stats, effects, and passive buffs. In order to unlock a constellation, you will need to obtain a Stella Fortuna material that can only be obtained from pulling a duplicate character via Wishes. This page covers a list of all the constellations found in each playable character in the game.
Genshin Impact Constellations