The Liberator, as stated in the first response, was an abolitionist newspaper written by William Lloyd Garrison. Given this basic definition there is a significant portion to understand about the iimpact this paper had on the era leading upto the United States' Civil War that needs further analysis. This paper was the primary item that got the once runaway slave Frederick Douglass involved in the abolition movement. Douglass, after reading this paper was motivated to put his very freedom and psubsequent possible return to slavery to join the the movement to end slavery in the United States. This paper not only motivated him but countless others to join in this effort. Additionally, at one point throughout the abolition movement Garrison will actually advocate for repatriation of all African Americans back to the continent of Africa in the Liberator and this would become one of many differences between Garrison and Douglass. This along with major difference between white and black abolitionists views, beliefs and understanding of what true emancipation was ultimately lead to a schism or sorts or their parting of ways. Douglass eventually started his own newspaper titled the North Star that had a reader base more intended for sympathizers of abolitionism from his perspective while Garrison's paper maintained it's past base of readers. In essence therefore is is necessary to not only know what the Liberator was but also to have a basic understanding of it's impact on the people of the time, such as Frederick Douglass.

I assume that you are asking this question about US history.  In that case, The Liberator was the name of an abolitionist newspaper that was started by William Lloyd Garrison (a famous abolitionist) in 1831.

Garrison was a believer in immediate and uncompensated abolition of slavery.  He did not agree with the idea of paying slaveowners for their slaves and he did not agree with the idea of sending the freeds slaves to Africa to live.

In order to disseminate his views, Garrison created The Liberator.  This was a major step in making the views of the abolitionist community more widely known in the US.