She plans on meeting Dimmesdale in the woods outside of town, where he will pass coming back from visiting another town. She does this because of the nature of the conversation she is going to have with him. Even though she knows no one in town would ever suspect the truth of their relationship (because of their reverence for Dimmesdale), Hester is fearful to discuss Chillingworth or their affair anywhere except under the canopy of the woods. This setting is significant to both theme development and symbolism. The darkness of the woods is representative of hidden truth. Also, independence and freedom are connected to nature throughout the novel, such as the rosebush growing on the prison wall and Pearl's playfulness in the field and by the brook.
Hawthorne also suggests that Hester wouldn't want to meet Dimmesdale in his study, which would be the most appropriate place for his parishioner to meet him, because she fears that Chillingworth may be about, and wants to convey her secret knowledge of Chillingworth to him.
"...she dreaded the secret or undisguised interference of old Roger Chillingworth..." and since her discussion is to be about how Roger is harming him, (...make known to him the true character of his intimacy...") prudence would suggest she do it in secret.
Hester planned to meet Dimmesdale beside a path leading through the forest. Because of the nature of their intended discussion, it was important that they meet out of range of prying eyes or ears. Secrecy was paramount to the livelihoods of both Hester and Dimmesdale, as well as Pearl. If someone were to happen across the two adults in conversation regarding their feelings for one another or Dimmesdale's fatherhood, serious repercussions would be almost inevitable; they needed to meet in a place that would allow them to be open and honest with one another.