What did Wendell Phillips mean when he said, "Revolutions never go backwards."? I believe that Abraham Lincoln also used this quote as a means of promoting the Republican Party and anti-slavery sentiment for the election of 1860.
In 1861, Wendell Phillips used this line, but he was quoting William Seward, not Lincoln.*
Seward used this line in his famous "Irrepressible Conflict" (full text here) speech of 1858. Seward was ardently anti-slavery, but also doubted the federal government had the legal power to mandate abolition. He therefore argued that the triumph of free labor over slavery was an ongoing revolution that could not be reversed, because modern states could only thrive with a free labor system. Seward meant for the speech to be moderate, but the South took much of the impassioned rhetoric to be tantamount to a declaration of war on their economic system.
Seward's speech was about the inevitability of freedom despite Southerners and Democrats dragging their feet. Ironically, Phillips echoes Seward to argue for the inevitability of freedom despite men like Seward dragging their feet. Phillips argued in his 1861 speech (in which, as elsewhere, he excoriates Seward) that the Civil War is merely the latest event in a centuries-long revolution for human freedom:
Our people stood, willing their idolized government should go to pieces for an idea. True, other nations have done so. England in 1640, — France in 1791, — our colonies in 1775. Those were proud moments. But to-day touches a nobler height. Their idea was their own freedom. Today, the idea, loyal to which our people willingly see their Union wrecked, is largely the hope of justice to a dependent, helpless, hated race. Revolutions never go backward.
There is a more detailed discussion of Seward's speech in the edit log, if you care to read it.
* The quote at least goes back to 1845 in a description of the French Revolution, but it is much more likely that Phillips meant to evoke Seward's famous speech.