What does a raven symbolize in the Bible?

Ravens symbolize that even what appears to be base, commonplace or repulsive can be holy.

Despite their seemingly unsavory habits ("The eye that mocketh at his father...the ravens...shall pick it out" Proverbs 30:17), and despite the fact that ravens are unclean food for Israelites according to the dietary prohibitions of Leviticus, ravens are fundamentally part of God's plan and therefore good ("every winged bird according to its kind...was good" Genesis 1:21). God's grace extends to ravens, and God "giveth...food...to the young ravens which cry" (Psalm 147:9). In fact, ravens are used by some of the heroes in the Bible--the raven is the first bird sent out by Noah to check if the flood has receded (Genesis), and God commands ravens to feed Elijah in the wilderness (1 Kings 17:4-6).

Several species of ravens are common in Israel, and the ancient Israelites would have been very familiar with them.

First Noah sent out a Raven and according to the bible,"

6 After forty days Noah opened a window he had made in the ark 7 and sent out a raven, and it kept flying back and forth until the water had dried up from the earth. 8 Then he sent out a dove to see if the water had receded from the surface of the ground. 9 But the dove could find nowhere to perch because there was water over all the surface of the earth; so it returned to Noah in the ark. He reached out his hand and took the dove and brought it back to himself in the ark. 10 He waited seven more days and again sent out the dove from the ark. 11 When the dove returned to him in the evening, there in its beak was a freshly plucked olive leaf! Then Noah knew that the water had receded from the earth. 12 He waited seven more days and sent the dove out again, but this time it did not return to him." Genesis 8:6-12