last: the final one in a sequence of order or time, the opposite of first
At its core this word marks the end of a line. Whatever comes at the close of a count, a list, or a stretch of time is the last one, set against whatever came first. The New Testament uses it for the final coin a debtor must pay, the youngest son a father sends, the most recent state of a person, and the closing act of a story.
From that plain idea grows the word's heaviest use. Stretched over the whole of history, the last day and the last days name the closing stage of God's plan: the time of resurrection and judgment, the season the prophets pointed toward, the era the first Christians believed they had already entered. The word does not by itself mean the end of the world; it means the final segment, and the context supplies what that final segment contains.
The risen Christ takes the word as a title, calling himself the First and the Last. Paired with first, it claims the two ends of everything: he is there at the beginning and waiting at the end, so nothing falls outside him.