Eid Al-Adha is the bigger of the two Eids of Muslims. The other Eid is Eid Al-Fitr, which marks the end of Ramadan and the beginning of Shawwal.
Eid Al-Adha marks the readiness of Prophet Ibrahim (peace be upon him) to sacrifice his son, Ismail (peace be upon him), when asked by Allah Subhanahu Ta’ala to do so. But Allah in His infinite mercy replaced Ismail with a ram with horns and it was sacrificed!