We use the description "words that hiss" to categorize the sounds that are difficult to say before a /s/ or /z/ sound. The simplest way to test if a phonogram "hisses" is to try saying these sounds after it and see whether you can say one of them easily.
You'll hear the "words that hiss" mentioned in Spelling Rule 21 and 22, which describe how to make nouns plural and how to make verbs 3rd person singular. Both specify you add -S "unless the word hisses or changes," and that in that case you add -ES.
This sounds trickier than it is, because when you are writing actual words you simply spell them the way you say them. Test this out:
- When you say the plural of a word, do you say a second syllable, /ez/, at the end (dresses, kisses, hatches, wishes, crutches, quizzes, boxes...)? If so, there's a second vowel in this word, so you're going to write -ES. The sound "hisses."
- Or, when you say the plural of a word, do you only say a /s/ or /z/ sound, with no second syllable, at the end (hats, kids, shoes, banks, slips, fans...)? If so, you're not going to add another vowel; you will only write -S.
The description "unless the sound hisses" is designed to help students characterize why these words are hard to say with S and therefore need a vowel sound added in between. But you and your students never need to memorize a group of sounds or phonograms or words that hiss, because it's easy to test. If you aren't sure, just try saying the word with /s/ at the end and see if it's difficult. Or say the plural and notice whether or not you add a new syllable, /ez/. Then write the word accordingly.