It's no secret to the rest of the Mane Six that Applejack has been crushing on Rarity for a long time. Despite being the Element of Honesty, however, Applejack has had trouble being honest with herself--about what she feels, and about where she'd like things to go with Rarity.
After a lot of soul-searching, Applejack has finally decided to tell Rarity how she feels, yet she's nowhere to be found. Only after taking a trip with Rainbow Dash does Applejack find out that Rarity has a secret too; quite a surprising one at that.

(Pic unrelated, just thought it was cute.)