In The Sun Also Rises, what is the significance of the title?

The title of Ernest Hemingway's first book is The Sun Also Rises, which comes from a verse in the Bible. The title is an apt depiction both of the despair of the Lost Generation of which Hemingway was a part as well as the potential for optimism in the perpetual rising of the sun. 

Ecclesiastes chapter one begins this way:

The words of the Preacher, the son of David, king in Jerusalem. Vanity of vanities, saith the Preacher, vanity of vanities; all is vanity. What profit hath a man of all his labour which he taketh under the sun? One generation passeth away, and another generation cometh: but the earth abideth for ever. The sun also ariseth, and the sun goeth down, and hasteth to his place where he arose. The wind goeth toward the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to his circuits. All the rivers run into the sea; yet the sea is not full.

These seven verses paint a picture of despair in which whatever one does is as nothing. Working (laboring) yields nothing, the winds are capricious, and the seas are never full. despite the waters continually pouring into them. The one constant is the earth, as demonstrated by the sun rising, setting, and rising again no matter what else is happening. 

Hemingway was part of what is called the "Lost Generation," a group of expatriate writers and artists who found real meaning in nothing as they spent their time reveling in their sinfulness while living in Europe. The picture of despair surrounding verse five (the sun also rises reference) is typical of what he and the others who were living this life felt, disillusioned by the materialism of post-war America. 

While the context of the verse/title is despair, there is also hope. Though everything seems hopeless, the sun will rise again tomorrow, and then it will do so again the next day...and the next. There is not much hopefulness in this novel, as the final words indicate:

“Oh, Jake,” Brett said, “we could have had such a damned good time together.”
Ahead was a mounted policeman in khaki directing traffic. He raised his baton. The car slowed suddenly pressing Brett against me.
“Yes,” I said. “Isn’t it pretty to think so?”

Despite that hopelessness about what might have been (but never really could have been), there is a glimmer of hope for the future of the Lost Generation in the rising of the sun.