ثُلَاثِيًّا مُتَحَرِّكَ الْأَوْسَطِ، كَـ«شَتَرَ»
Translation: A three-letter form whose middle letter is vowelled, as in Shatar.
شَتَر has three consonantal letters, ش ت ر, with a vowel on the middle ت. The author presents it as an example of a foreign proper name.
The purpose is grammatical: it illustrates the three-letter, vowelled-middle alternative, not a vocabulary word to memorize.