What makes a story “decodable”?
Early reading is usually taught in a sequence: first the sounds of single letters and short words like cat and pig, then blends like fr and st, digraphs like sh and ch, then silent-e words like cake, and so on. A decodable story uses only the patterns a child has reached in that sequence. A few words that can’t be sounded out yet, like the and said, are taught separately and learned by sight; many programs call them heart words, because part of them has to be learned by heart.