Hacktoberfest happening in the month of October is a celebration where it encourages people to work on open-source projects run by DigitalOcean in partnership with DEV.
According to their website
Hacktoberfest is open to everyone in our global community. Whether you’re a developer, student learning to code, event host, or the company of any size, you can help drive the growth of open source and make positive contributions to an ever-growing community. All backgrounds and skill levels are encouraged to complete the challenge.
The rules are simple, you just need to create 4 PRs (Pull Request) in any of the open-source repo and thus you are entitled to receive limited edition Hacktoberfest t-shirt. Do note that only the first 50,000 participants who successfully complete the challenge will get a t-shirt.
Once you complete 4 PRs, you need to wait for another 7 days for the review period. Once the review period is over you will be officially complete the challenge to get a t-shirt and swag.
Many companies joined the celebration by rewarding their contributors with even more swag which can be viewed here.
Now the question arises why you should contribute, it's not only for the t-shirt you will earn. We use a lot of open-source software in our daily life (steem being one), by contributing we are just giving back to the community.
If you want to start and looking for a project, do let me know. I will help you find a project.