The simple answer is that Mrs. Reed has concealed a letter she received from John Eyre, Jane's uncle, who wished to adopt Jane and bequeath her money. Mrs. Reed wishes to give the letter to Jane.
More important is that Mrs. Reed is a member of the Evangelical wing of the Church of England. One cannot die in a full state of grace without being reconciled to ‘God and neighbor’. Thus if Mrs. Reed had died without being reconciled to Jane, she would (in her own belief system) be condemned to go to Hell. Thus by obtaining Jane’s ‘full and free forgiveness’ , Mrs. Reed is preparing herself for the judgement she believes she will face after death.