Machinable Mail is mail that can go through the sorting machine easily but non machinable mail cannot go through the sorting machines to be read and sorted.
Machinable mail will be able to go through the sorting machine where it can be read by the machine and computer and go through to the proper spot to where it can be delivered to the correct address.
A non machinable piece of mail would be mail such as a letter that has something inside it other than paper such as a pen or pencil, key ring etc.
The item inside the letter will make the envelope be irregular and bulge out which makes it non machinable so then a human has to hand sort it and they will stamp it as non machinable.
Non machinable mail usually requires a bit more postage than a standard letter would require.