What does 'git commit' mean when it says 'create mode ...' on stdout?

These are the file permissions as unix-style permissions values. They're printed in octal and represent clusters of 3 bits for read, write and execute. If you look at a tree object in git (eg: git ls-tree HEAD) you can see everything git records about the contents of a directory. That is tree contains trees and blobs with the permissions bits

C:\project>git ls-tree HEAD
100644 blob 66f3f25c8ca9ae73b99669aca6ba5ecfa4703b2b    .gitignore
100644 blob 60b88ac20b8b7cccdcd856e65415a9eb9495b63a    Makefile
040000 tree e1d9381e4d12effea7e33f8d7e2b16e372f67b51    demos
100644 blob a60e08eeb9f75160ae2bf6a9feeff3c1c75bfc1d    doxygen.cfg

6 means read-write, 4 is readonly.

For more information about Git's mode, see this answer.

Git's ability to store file metadata is limited to a simple subset of information to allow Git to track some basic file system changes allowing Git to track relevant changes for source code management; such as whether a file has been modified and whether a file is a regular file or an executable file.

Git does not try to implement any notion of a file system, leaving file system routines to an actual file system implementation. This makes good sense to allow Git to work equally whether on a FAT32, NTFS, EXT3, XFS, NFS, etc. file system running on Linux, MacOS, Windows, etc.