In Ubuntu filesystem, there is a
/ directory which is called "the root directory" and in that
/ directory there is a directory literally named
root. What is the difference between them?
/ is the topmost directory in the file system. All other directories are subdirectories of it at some depth.
/root is the home folder for the 'root' user (see
id root and What is the root user?). The purpose of this directory is to hold user configuration files for root. Usually you won't find much inside it.
Both are sometimes confusingly designated by the word "root". If you imagine the filesystem as a tree upside down, it grows from the root. The name of the root user might have come about with reference to the name of the root directory.