what are Biter and Beater? Why does the fact that thorin has Biter make the Great Goblin fly into a rage? How does gandalf use Beater to escape from the goblins?

Biter is a sword. Ot had been used to kill hundreds of goblins.

"The Great Goblin gave a truly awful howl of rage when he looked at it, and all his soldiers gnashed their teeth, clashed their shields, and stamped. They knew the sword at once. It had killed hundreds of goblins in its time, when the fair elves of Gondolin hunted them in the hills or did battle before their walls. They had called it Orcrist, Goblin-cleaver, but the goblins called it simply Biter. They hated it and hated worse any one that carried it."

Beater was also a sword, Gandalph used its bright blue light to help them escape.

"Then Gandalf lit up his wand. Of course it was Gandalf; but just then they were too busy to ask how he got there. He took out his sword again, and again it flashed in the dark by itself. It burned with a rage that made it gleam if goblins were about; now it was bright as blue flame for delight in the killing of the great lord of the cave. It made no trouble whatever of cutting through the goblin-chains and setting all the prisoners free as quickly as possible. This sword's name was Glamdring the Foe-hammer, if you remember. The goblins just called it Beater, and hated it worse than Biter if possible. Orcrist, too, had been saved; for Gandalf had brought it along as well, snatching it from one of the terrified guards. Gandalf thought of most things; and though he could not do everything, he could do a great deal for friends in a tight comer."