A symbol is any object that has a second wider meaning that it stands for apart from its own literal meaning. In this excellent play, the main symbol to focus on is the bird that Mrs. Hale and Mrs. Peters discover in the box of Mrs. Wright's sewing things. We are told by Mrs. Peters, "in horror," that somebody had wrung the neck of this beautiful bird that used to sing. Whilst Mrs. Peters and Mrs. Hale recover from the shock of finding this bird, they talk and surmise about what had happened to it. Note what Mrs. Hale says:
No, Wright wouldn't like the bird--a thing that sang. She used to sing. He killed that, too.
We can see from this quote and others that refer to what Minnie Wright was like before she married that the bird is being used as a symbol of Minnie:
She--come to think of it, she was king of like a bird herself--real sweet and pretty, but kind of timid--and fluttery. How--she--did--change.
The bird has been slowly strangled to death by John Wright, just as symbolically Minnie Wright has been slowly strangled to a kind of death in her life. It is this that the two women realise, having discovered the motive for the crime in the "trifles" that the men discount, and in an act of female unity, they decide to hide the bird--the symbol of Minnie's guilt--and cover up the crime.