The values shown are the 16-bit file modes as stored by Git, following the layout of POSIX types and modes:
32-bit mode, split into (high to low bits)
4-bit object type
valid values in binary are 1000 (regular file), 1010 (symbolic link)
and 1110 (gitlink)
9-bit unix permission. Only 0755 and 0644 are valid for regular files.
Symbolic links and gitlinks have value 0 in this field.
That file doesn’t mention directories; they are represented using object type 0100. Gitlinks are used for submodules.
Each digit in the six-digit value is in octal, representing three bits; 16 bits thus need six digits, the first of which only represents one bit:
1000 000 111101101
1 0 0 7 5 5
1000 000 110100100
1 0 0 6 4 4
Git doesn’t store arbitrary modes, only a subset of the values are allowed, from the usual POSIX types and modes (in octal, 12 for a symbolic link, 10 for a regular file, 04 for a directory) to which git adds 16 for Git links. The mode is appended, using four octal digits. For files, you’ll only ever see 100755 or 100644 (although 100664 is also technically possible); directories are 040000 (permissions are ignored), symbolic links 120000. The set-user-ID, set-group-ID and sticky bits aren’t supported at all (they would be stored in the unused bits).
See also this related answer.