what does "bitcoin days destroyed" mean exactly???
It's really simple once you "get it" but it took me a while to wrap my head around it too. I'll try:
If you send me 1 BTC and I spend it tomorrow, when I spend it, I will have destroyed 1 bitcoin-days.
If I wait till next month to spend it, when I spend it it will destroy 30 bitcoin-days.
But I you send my 60 BTC and I spend them 12 hours from now, it would also destroy 30 bitcoin-days.
So, it's a combined measure of the magnitude and oldness of the coins that are being spent. If we see a huge spike in the amount of bitcoin-days destroyed, it's a good indication that someone just moved a lot of old coins.
Interestingly: something akin to bitcoin-days is used by the bitcoin-network to determine the priority of a transaction. AFAIK:
priority = (# of BTC) * (age) / (size of TX in bytes)