In software with plugin capabilities, typically in interpreted languages, it is possible that plugin have dependencies included, that may collide with dependencies included in core product, or in other plugins. It is not a problem when the same version is used, but in turns into a problem when different versions that are incompatible to each other are shipped. The end user may face bugs. Developers speak of the Dependency Hell.
In Nextcloud these plugins are the various apps, coming from various sources.