What on earth are Red Words anyway? Sight words? High-frequency words?

For us, Red Words are phonetically irregular words. They are quite simply words that can't be sounded out. If you sound them out they come out all wrong. "Was" for example is a Red Word. If we sounded this word out based on syllable type (extra points for you if you knew it would be a closed syllable) you would get /w/ /a/ /s/ which would rhyme with gas. Makes sense right? But instead, we get a lazy/schwa vowel in there and get a short /u/ sound. So we just have to memorize these words for both reading and spelling.