Huh? Verse 5 in the NASB reads:
"Now I desire to remind you, though you know all things once for all, that the Lord, after saving a people out of the land of Egypt, subsequently destroyed those who did not believe."
However, the Greek word for "Lord" in this verse is Ιηοῦς and the ESV has translated this as such.
"Now I want to remind you, although you once fully knew it, that Jesus, who saved a people out of the land of Egypt, afterward destroyed those who did not believe."
Why is this important?
William Mounce explains at Koinonia. It's a short article but worth the read. He is an excellent scholar and a trusted source for this kind of interpretation.