The fairy Puck has made a horrible mess of things with his love potion. Just a little juice from his flower has turned Lysander away from his true love, Hermia, and made him chase after her best friend, Helena. Demetrius, too, has receive a bit of the potion, and he too is now madly in love with Helena. Helena thinks all three of them are mocking her. Hermia is just plain confused and hurt. The passage in question dramatically expresses these strong emotions that pass back and forth between these friends-turned-lovers-turned-enemies.
Poor Hermia has no idea what is going on. She hears Lysander's voice and is relieved to find him, only to have him spurn her and tell her that he hates her and is in love with Helena. This is all the more shocking because Hermia and Lysander are out in the woods because they are on their way to his aunt's house to get married. Hermia argues that he cannot possibly think like that.
Helena is actually getting quite angry at this point. She doesn't understand why she has two men chasing her. All Demetrius has done prior to this is turn away from her, for he has been in love with Hermia. Helena loves him, but her love has always been rejected. As for Lysander, his behavior is totally out of character. The only explanation Helena can think of is that they are playing some kind of nasty joke on her and that her best friend is involved, too. She lashes out at Hermia quite cruelly, accusing her of betrayal despite their long unity. Everything must be Hermia's fault, Helena thinks. It must be Hermia who has set both men to this mocking.
This little speech reveals the jealousy that Helena holds in her heart. Hermia has a true love in Lysander, yet Demetrius, the man Helena loves, has also declared his love for Hermia. Helena is left out completely, and while she has tried to remain positive, this is the last straw. She declares that the whole thing has been partly her own fault for caring about any of them.
In these lines, then, Shakespeare shows how emotions can get out of hand, how rivalries can break out, and how friendships can falter. Certainly Puck's interference hasn't helped, and he must fix his mistakes, but the characters themselves are going to have to face their deepest feelings before they can sort out their relationships.