How is June a foil to August and what is the purpose of a foil to the plot?

The purpose of using a foil in a plot is to highlight the good qualities in one of the main characters, and truly allow those qualities to shine.

In the 'Secret Life of Bees,' August's character is warm and loving; she's independent, productive, educated, and best of all, she has a caring instinct a mile long. June serves as her sister's foil; she resents August's concern for people in the white world, and she resents that her sister works as a housekeeper for a white employer. Thus, the reader looks at June's character and thinks August all the more commendable for her actions.

In defense of June....... she eventually comes around, and her hard edges soften up.