how do the symbols introduced in the setting give the readers more insight into the storyline

There is Larry's Tree, built as a memorial when he didn't return from WW2,

"In the left corner, downstage, stands the four-foot-high stump of a slender apple tree whose upper trunk and branches lie toppled beside it, fruit still clinging to its branches."

The tree serves as a polarizing symbol to the Keller family. Mother, for example, uses it as a symbol that Larry will still return while Chris chops it down because he wants no reminders of his brother. Chris wants to marry his brother's widow, Ann.