Phantom Dancer - Lament

i know about the first passive where you get ms if there are enemy champion near you. but i dont understand lament passive.

Grants 12% damage reduction against the last enemy champion hit (ends after 10 seconds out-of-combat). what does this mean? Also what does death defied mean? when does number go up/down?

thank you so much if u can answer all of the questions for me <3