It refers to the year around 570 CE in which Abrahah al-Ashram, the Abyssinian ruler of Yemen, marched with an army that included an elephant intending to destroy the Ka'bah. Allah destroyed his army by sending flocks of birds that pelted them with stones of baked clay, as recounted in Surah al-Fil. The Prophet Muhammad (صلى الله عليه وسلم) was born in this same year, and the event stands as a sign of Allah's protection of His Sacred House even before the coming of Islam.
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.