First page of Chapter 3 under the Branches in a Nutshell section 5th paragraph:

"When you create the commit by running git commit, Git checksums each subdirectory (in this case, just the root project directory) and stores those _tree_ objects in the Git repository."

Not sure if you really meant "tree" or "three".

(above message is copied from git-scm.com issue 601)

File: book\03-git-branching\sections\nutshell.asc
Line: 20

In my understanding, it talks about tree object.
Is this good enough?

Git checksums each subdirectory (in this case, just the root project directory) 
and stores them as tree object in the Git repository.