Abraham Lincoln is important for two main reasons.
First, Lincoln is important because of his role in freeing the slaves. Lincoln was not an abolitionist or a believer in racial equality. He is, therefore, not as morally pure (with regard to race and slavery) as we sometimes portray him. However, he was anti-slavery, perhaps more than any other major political figure, and he did help to end slavery through the Emancipation Proclamation when it was not completely necessary or inevitable that he do so.
Second, Lincoln is important because of his role in the Civil War. Lincoln is the figure who gets the most credit for seeing the Union through this crisis. He showed the moral courage to refuse to allow the South to secede. He chose, instead, to go to war against them. During the war, it was Lincoln more than anyone else who allowed the North to succeed. His political skills helped hold the country together and he ended up being more of a military leader than might have been expected.
Thus, Lincoln allowed the US to emerge from the Civil War as a country that was much more united and much more just than it had been before.