It means that these bitcoins will never be moved again.
1 The initial bitcoins mined and tested by Satoshi Nakamoto
It is said the amount is up to 1 million BTC.For now there is no evidence that Satoshi Nakamoto sold or moved these bitcoins,he only moved 18 BTC which was used to test.Basically we believe that he will never move these BTC again.
2 Those bitcoin wallets lost private key
The situation should be normal at the beginning,For example,someone mined some BTC for fun then abandoned it and even delete the bitcoin client.They do not care these btc because the market price is very cheap at that time.
3 some bitcoins destroyed deliberately
For example: someone destroyed some BTC to get some next generation coin
what is the amount of these bitcoin in total?They should be removed from the market capitalization,but no one knows the exact amount.