Joseph Smith attempted a translation of the Bible. He, like many interpreters before him, sought to iron out the parts that didn't quite make sense.
So in Hebrews 6:1, he changed "Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection..." to read "Therefore not leaving the principles of Christ, let us go on unto perfection..."
Now, I'm not 100% sure what the author intended to say with this verse (it seems to me he was saying "let's not talk about the stuff we already know about Jesus, the basic stuff"), but it seems pretty clear Joseph slapping the word "not" in there as a means to fix what he perceived was a contradiction raises more issues than it fixes, and shows to me he didn't know exactly what the verse was saying, either.
One issue it raises was how inspired Joseph was when he did his translations. If he was wrong or misguided in translating the Letter to the Hebrews, it opens up questions regarding his other translations.
The Mormon footnote in the King James Version doesn't seem to agree with the Prophet, either.