That is a git submodule directory.
Submodules allow you to keep a Git repository as a subdirectory of another Git repository.
If you have the repository downloaded locally you should be able to browse the last version of the submodule. On Bitbucket Cloud, the directory will link to the source. If the link isn't working the source may have been moved or deleted.
Try running this in the root directory:
git submodule
Here is some further reading: https://git-scm.com/book/en/v2/Git-Tools-Submodules
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