Literally, Bertha Mason is a very real impediment to Jane's marriage to Mr Rochester. Since he is still married to her, his marriage to Jane cannot progress as planned.
As a metaphor, Bertha represents the gothic element in the novel - wild, dark, closely related to nature. In addition, we can also see her as the 'madwoman in the attic.' This is a concept that was developed by Sandra Gilbert and Susan Gubar in their 1979 book examining Victorian literature from a feminist perspective. This work suggests that madness in Victorian writing was often an analogy for rebellion and anger against the restrictions placed upon women in the home and in public. The result of this for Jane is that it forced her from Thornfield, gave her the opportunity to re-discover her family, stand as an independent woman and not marry just because it was offered to her. When she returned to Rochester, it was on her own terms.