Following his death, Abraham Lincoln became known as the Great Emancipator for his efforts in ending slavery in America. His presidency coincided with the Civil War, and when Lincoln delivered his Emancipation Proclamation speech, he changed the federal legal status of an estimated 3.5 million African Americans from slave to free. And for many years, he was greatly lauded as a hero for his efforts.

However, his title as the Great Emancipator has been questioned in more recent years as historians have taken a closer look at his beliefs. Many believe that the Emancipation Proclamation was more of a political strategy than a personal one; that is, they believe that he was more interested in ending the Civil War than in securing freedom for enslaved people. Even former President Barack Obama has said, "I cannot swallow whole the view of Lincoln as the Great Emancipator.” He went on to comment that at the least, President Lincoln did address the issue of slavery in a straightforward manner rather than allowing it to linger for future generations to contend with.

Regardless of his personal intentions, the Great Emancipator did change the trajectory of the future for African Americans, though their civil struggles were far from over once the speech was delivered.

Answers on this site are recommended to be of 90 words, so I'll try to stick around there and give you a basic foundation that you can build on.

To "emancipate" means to set free.  Lincoln is known as "The Emancipator" because it was during his time as president that the slaves in the United States were freed.

Lincoln can be credited with freeing the slaves in two ways.  First, he chose to fight a civil war against the South instead of allowing them to secede.  If they had seceded, they would have been able to keep their slaves.  By fighting, and winning, the Civil War, Lincoln allowed the slaves to be freed (this was done by the 13th Amendment to the Constitution, ratified after the war).

The second thing Lincoln did was to issue the Emancipation Proclamation, which went into effect on January 1, 1863.  This order freed all slaves in areas that were then rebelling against the US.  Although this order did not free all the slaves, it established the idea that the Civil War was to have freeing the slaves as one of its major goals.