But it does say what it means. It says:
“But, beloved, do not forget this one thing, that with the Lord one day is as a thousand years, and a thousand years as one day.” (2Pe 3:8 NKJV)
Two things are clear. It's "with the Lord" that what is being spoken of is true. And with the Lord, one day is as (that is, like) a thousand years, and a thousand years are like a day. It doesn't mean that those things are true for us. Nor does it say that with the Lord a day is a thousand years, but that it is "as a thousand years." God is outside of time so whether it's a thousand years or one day makes no difference to Him.
Incidentally, where in the bible are we told that it will be 6 days/6 thousand years between Noah stepping out of the Ark and Jesus Christ's coming?