Hacktoberfest is an annual event that encourages participation in open-source software development. It takes place in October and is sponsored by DigitalOcean and GitHub. Participants are encouraged to contribute to open-source projects by submitting pull requests on GitHub, and in return, they can earn rewards such as limited-edition T-shirts and other swag. It’s a way to promote and celebrate open-source software development and collaboration within the global developer community.