It means the seed is jam packed full of starch to give the seed energy to grow.

Like your lunch box, when momma didn't pack a pudding cup and shorted you on cold cuts it will be light and you won't have enough energy.

Probably means nothing.

https://www.gardenmyths.com/floating-seeds-in-water/

If you want to test your seeds, put them in a damp paper towel in a plastic container/baggie for a week or two. You can do a bunch at once, if you label your towels with a permanent marker. My experience is that 3 year old seeds will germinate just fine. I wouldn't even bother to test them. I've sown 10 year old seeds of various veggies and flowers pretty heavily and got a decent germination. (Since I sow it heavily, I really don't know whether 20% or 80% of the seeds germinate.)

I have nver had them sink right away - I do know the test is iffy and I have done the germination text with a wet paper towel. I am getting more seeds, I was just surprised then sank immediately! I have done this a dozen times of more and that never happened.