Twenty high-frequency words that make up a large share of anything a beginner reads. Most of them cannot be sounded out — the, is and you break the usual rules — so they are learned whole, by sight, rather than decoded letter by letter.
Common words a reader recognises instantly rather than sounding out. Many are irregular, so memorising them is faster than decoding them.
Most reading schemes aim for twenty to fifty by the end of kindergarten. The twenty here are among the most frequent in early books.