Board Thread:Dark Souls II/@comment-183.90.118.193-20141022065738/@comment-3492791-20141022072108

This is explained in the first Dark Souls.

"The flow of time itself is convoluted; with heroes centuries old phasing in and out. The very fabric wavers, and relations shift and obscure. There's no telling how much longer your world and mine will remain in contact. But, use this, to summon one another as spirits, cross the gaps between the worlds, and engage in jolly co-operation!"

- Solaire of Astora