Jane Eyre tackles prejudice and elitism within British society, but the madwoman in the attic (Bertha) is something of an enigma. Rochester can see beyond Jane's poverty, and finds culture in Jane (she is poor but she has been well trained) but he cannot see beyond his wife's emotions. 

Coming from the Bahamas, Rochester's wife Bertha has gained his revulsion because she has an excess of passion. He sees this as evidence of her savage nature and of her ultimate lack of civility. She wants sex, she gets angry--very angry, she lacks all restraint and decorum. Rochester does not deal well with her at all--he has her locked up and calls her insane. By the time we (readers) meet her, she undoubtably is. But Bertha has qualities that seem quintessentially female, and many 'modern' readers, especially those women who see in her our own potential imprisonment (metaphorically speaking), have found her the most interesting character in the book. The Wide Sargasso Sea is a character study as well as a commentary. It gives the madwoman a voice and a history.

Wide Sargasso Sea is a response to Charlotte Bronte's Jane Eyre.  It is important because it presents a completely different perspective of the character of Edward Rochester's first wife. It also explores issues of postcolonialism and specifically the relationship between the West Indians and the English in the post-emancipation Caribbean, exposing the attitudes which allow the domination of one race over another. 

In Jane Eyre, Rochester's wife Bertha is depicted solely as a madwoman, subhuman, a dangerous and promiscuous monster locked away in the attic, and Rochester as the innocent and unhappy victim of an arranged marriage.  In Wide Sargasso Sea, Bertha is called Antoinette, a real person with needs and motivations for the things she does.  A child of mixed race, Antoinette has been an outcast all her life.  Although her marriage with Rochester is indeed arranged by others, it is his rejection and revulsion to her sensuality and mixed heritage which drives her to the brink of insanity.