in the root project 's build.gradle:
I do not know meaning of that, and I could not find the meaning.
What does root project mean?
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I assume that the documentation is discussing a hierarchy of projects. In other words, one project can contain other project(s). There might even be multiple levels of projects within projects within projects. In computer programming, this kind of structure is called a "tree". A "directory tree" is a similar structure.
A real-life tree has a trunk, which has branches, which have branches, which have twigs, which have leaves. At the bottom of the trunk are the roots of the tree.
By analogy, the directory that contains all of the other directories in a directory tree is called the "root directory".
In your example, the project that contains all of the other projects is called the "root project".