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.
Because he wrote and passed the Emancipation Proclamation. Emancipation means "freedom;" The Emancipation proclamation ends slavery...kind of.
The Emancipation Proclamation is often misunderstood - it states that slaves in REBEL states are freed. However, the very fact that they were states in rebellion against the Federal Government means that they weren't listening to Lincoln, and so they did not free their slaves. It also meant that any slaves in the border states (MD, KY, MO, DE, WV) would not be set free (how much does that suck for them?!). So what does the Emancipation Proclamation do? Nothing really. It does, however, change the purpose of the war from a fight to save the Union to a fight to end slavery.
Slavery was not officially ended until the 13th Amendment was passed, and Lincoln was already dead by that point. However, he is still credited as being the President who ended slavery - hence being called "the great emancipator."
He freed the slaves then very prominent in America and brought about the Emancipation Proclamation.