There are two main symbols in The Story of an Hour. The first is the heart trouble: Louise's physical problems with her heart are symbolic of the unhappiness she feels in her marriage and her lack of freedom.
The second symbol is that of the open window. It represents the freedoms that she can enjoy at her husband's death. When she turns away from the window, she is turning away from her freedom and hope.
The irony is that her heart problems end up causing her death. The doctors say she died of joy upon seeing her husband again, but in reality she is dying of sadness because she has lost the freedom that she was so close to getting.