When you add a package to a Project manifest, Unity considers that package a dependency of the Project. However, a package can also have dependencies on other packages, which might create indirect dependencies in a Project that uses this package.
In this example, if you install Package A and Package B in your Project, then your Project has direct dependencies on both Package A and Package B. However, if Package B also has a dependency on Package C, then your Project also has an indirect dependency on Package C.
A conflict exists when a Project has dependencies on a package with different versions. A conflict can only exist between two indirect dependencies, when neither is added explicitly in the dependencies of the Project’s manifest file.