English Idiom: Birds of a feather flock together.
Explanation: Just as bird species flock together, people with similar interests tend to congregate with one another.
There are a lot of technical and management subjects that need attention, but not all of these can be planned as a workshop. Birds Of a Feather, or BOF sessions, bring people with similar interests together to meet and discuss these topics. The topics are agreed by the attendees on the first day of the conference and then the session times and locations are publicised.
For example, one BOF session might be a group of people discussing the possibility of using open source software in their mission organization, and another could be discussing the implications of data protection laws on how they store and use data.