Helena is referring to a famous myth involving the god Apollo, and Daphne, a forest dwelling nymph. 

One day, Apollo teases Cupid for being just a boy-god with a toy bow and arrow. Cupid retaliates by shooting Apollo with a gold arrow, causing him to fall in love with Daphne. He then shoots Daphne with a lead arrow, causing her to detest Apollo. When Daphne rejects Apollo's attempts at seduction, he gets frustrated and chases her through the forest. Just before he’s about to catch her, she calls out to her father (a minor god) for help. He saves her by turning her into a laurel tree. Despite having his sexual aggressions thwarted, Apollo decides to honor Daphne by holding the laurel tree in high esteem, and that’s why laurel wreaths have been used to crown victorious generals and athletic champions ever since.

Helena notes that in her relationship with Demetrius the situation is ironically reversed: it's the woman who is pursuing the man through the forest.

(Apollo and Daphne by Piero del Pollaiolo, circa 1475)