This is a pretty broad question, but I think that the best brief answer is that Elizabeth Cady Stanton was one of the major figures who started the movement for women's rights in the 1840s.
Like many of the people who were to get involved in the movement for women's rights, Stanton had been an abolitionist. However, she came to believe that there was a need to fight for the rights of women, not just those of slaves.
As a way to get this movement started, Stanton composed a Declaration of Sentiments. She presented this declaration at the famous meeting at Seneca Falls, in New York State, in 1848. This meeting is seen as the start of the women's rights movement. For this reason, Stanton is seen as one of the earliest leaders of the movement.