Sri Ramachandra Vijayottsava or Dussehra is celebrated to honor the victory of Lord Ramacandra. Lord Ramacandra is an incarnation of the Supreme Personality of Godhead or Sri Krishna.
Lord Ramachandra killed a Demon named Ravana who had kidnapped Lord Ramachandra’s eternal consort Sita Devi. Lord Ramachandra had asked Ravana to release Sita Devi, but Ravana had refused. The situation escalated to such an extent that it led to war. Lord Ramachandra finally killed Ravana, so religiosity rose to power again. That is, the celebration represents the victory of good over evil.