What does this quote mean the blood dimmed tide is loosed?

"Turning and turning in the widening gyre

The falcon cannot hear the falconer;

Things fall apart; the center cannot hold;

Mere anarchy is loosed upon the world,

The blood-dimmed tide is loosed, and everywhere

The ceremony of innocence is drowned;

The best lack all conviction, while the worst

Are full of passionate intensity.

Surely some revelation is at hand;

Surely the Second Coming is at hand.

The Second Coming! Hardly are those words out

When a vast image out of Spiritus Mundi

Troubles my sight: somewhere in sands of the desert

A shape with lion body and the head of a man,

A gaze blank and pitiless as the sun,

Is moving its slow thighs, while all about it

Reel shadows of the indignant desert birds.

The darkness drops again; but now I know

That twenty centuries of stony sleep

Were vexed to nightmare by a rocking cradle,

And what rough beast, its hour come round at last,

Slouches towards Bethlehem to be born? "

This is a disturbing poem about the end of the world. The line that you are asking about, "the blood-dimmed tide is loosed, and everywhere the ceremony of innocence is drowned," evokes the image of bloody water flooding over innocent people and drowning them. You might use this quote if some particularly horrible situation has just begun - war or massacre, for example.