Hi, this event allow you to contribute to Joplin and, at the end of the month, after having done 5 PR, you'll earn a limited edition T-shirt.
To participate go on https://hacktoberfest.digitalocean.com/ log in (with you github account) and you are ready to get in
Next, go dive into the Joplin issues list labelled “Hacktoberfest”
We hope you will enjoy that event by contributing to the project which is a nice moment of sharing good vibe
PS: the 5 Pull Request don't have to be done only on Joplin project, those can be done on any FOSS projects.