Subhadra was a very beautiful daughter of Vasudev and she was sister of Balram and Krishna. Arjuna during his penance exile saw Subhadra during a gathering of Vrishnis and Andhakas and immediately fell in love with her. With Krishna’s advice, he abducted Subhdra when she was doing prayers. The warriors of Dwaraka got really angry but Krishna with his words convinced the warriors including Balram to accept the marriage and welcome Arjuna. Arjuna and Subhadra were welcomed in Dwarka where they ritually got married. All this happened with blessings of Shri Krishna because Arjuna would never do such a thing to a sister of his best friend who was also equivalent of his Guru without his permission. The feelings of Subhadra is not mentioned in Mahabharata regarding her abduction but she seems to have happily accepted Arjuna as her husband.
Draupadi initially was salty with this relationship, probably she thought Subhadra was her equal and therefore her rival but on Arjuna's orders she introduced herself to Kunti and Draupadi as a milkmaid and her humility won over Draupadi and she accepted Subhadra as her co-wife.
Source- Mahabharata