Theseus is the duck of Athens. He is engaged with Hippolyta and he is marrying her in 4 days. He is trying to convince Hermia to marry Demetrius because that is who Hermia's father wants him to marry.
Hermia is a young woman who is in love with Lysander but her father chose her to marry Demetrius. The law is to marry who your father choses but Hermia does not like him. He is playing to go out of the limit of where the law applys to marry Lysander
Demetrius is currently engaged with Hermia. Helena is in love with Demetrius but he does not like her. Hermia does not like Demetrius but her father chose Demetrius to marry her and she has to follow that.