Puck puts the juice of the flower into Lysander's eyes on accident. Puck is a servant of Oberon, the king of the fairies. Oberon is in a fight with his wife Titania over who gets to keep a certain servant. Oberon tells Puck to use the juice of the flower called love-in-idleness on Titania's eyes to make her fall in love with someone so she gets distracted from the fight.
Oberon also tells Puck to use the flower on Demetrius. Demetrius is in love with Hermia, but Hermia is in love with Lysander. Hermia and Lysander have run into the woods where Oberon lives to get married. Demetrius followed them. And Helena, who is in love with Demetrius, follows him. Oberon sees them interact, knows the Helena loves Demetrius and wants to make Demetrius love Helena. He tells Puck to use the flower on Demetrius so he'll fall in love with Helena. But Puck makes a mistake and uses it on Lysander instead.
Lysander tells Helena that he is in love with her. She thinks that he is making fun of her because she knows that he is in love with Hermia. She gets angry and thinks that both Hermia and Lysander are mocking her because she loves Demetrius.