An ancient people group
The Rephaim first appear in the days of Abraham. Chedorlaomer and his allies defeated them at Ashteroth-karnaim, a place east of the Jordan (Genesis 14:5). Scripture does not give a full family history or origin story for them. It treats them as one of several established peoples of the region.
Later passages place Rephaim in Bashan and also connect them with territory that had formerly belonged to the Ammonites. Deuteronomy explains that the Ammonites remembered earlier inhabitants there as Rephaim, though the KJV calls them giants in that setting (Deuteronomy 2:20).
And in the fourteenth year came Chedorlaomer, and the kings that were with him, and smote the Rephaims in Ashteroth Karnaim, and the Zuzims in Ham, and the Emims in Shaveh Kiriathaim,
Why they are linked with giants
Deuteronomy says the Rephaim were considered giants, and compares them with the Anakim, another people known for their imposing size (Deuteronomy 2:11). The Bible does not pause to give modern measurements for the whole group. Its concern is that these were people Israel and its neighbours regarded as powerful and intimidating.
Og, king of Bashan, is the best-known Rephaite in Scripture. Moses identifies him as a remaining survivor of the Rephaim and notes the exceptional size of his iron bedstead (Deuteronomy 3:11). Og's kingdom was real, strong, and feared, yet Israel defeated him as it travelled towards the promised land (Joshua 12:4).
For only Og king of Bashan remained of the remnant of giants; behold, his bedstead was a bedstead of iron; is it not in Rabbah of the children of Ammon? nine cubits was the length thereof, and four cubits the breadth of it, after the cubit of a man.
The same word can mean the dead
Here the question becomes more interesting. The Hebrew term rephaim is also used in several poetic passages for the dead. In Isaiah's taunt against Babylon's fallen king, the realm of the dead is pictured as rousing its departed rulers to meet him (Isaiah 14:9).
That poetic use does not require us to conclude that the ancient Rephaim people were ghosts, or that every appearance of the word means exactly the same thing. Words can carry more than one related sense, especially in poetry. Read the verses around it: a historical account will be speaking about a people, while a poem about death may use the term for the departed.
Hell from beneath is moved for thee to meet thee at thy coming: it stirreth up the dead for thee, even all the chief ones of the earth; it hath raised up from their thrones all the kings of the nations.
How to read these passages
The Rephaim are not introduced to make readers anxious about hidden mysteries. In Israel's story, they help show the scale of the opposition in the land. The people faced nations that looked overwhelming, including rulers such as Og.
Still, the biblical account does not leave the focus on their reputation. It records their defeat and Israel's progress into the land (Joshua 12:4). When you meet the Rephaim in Scripture, notice the kind of passage you are reading and let its wider message guide the question.