HERMIA and DEMETRIUS are running through the wood.  DEMETRIUS is chasing HERMIA who is in love with LYSANDER and who has eloped to the woods to get away from DEMETRIUS, who her father wants her to marry.  Later in the scene, LYSANDER and HELENA stumble on the same clearing: LYSANDER is really in love with HERMIA, but PUCK’s love potion has caused him to have (temporary) feeling for HELENA.  HELENA – in turn – is in love with DEMETRIUS.