Probably for the small size you'd k1, and inc 1, but for the middle size you k1 and dec 1, and for the Large size you k1, dec 3. That would be to make the stitch counts turn out right for a stitch pattern that needs a specific number of stitches to work.
It's as I said - those are what you do depending on which size you're making. At the beg it says "Sizes - To fit 0–3 (3–6: 6–9) months" So you see the med and L size in parentheses with a : separating them like in the dast on - cast on 97 (112: 127) sts. So depending on which size you're doing when you get to that part you k1, increase 1 OR k1 dec 1, Or k1, dec 3 sts evenly spaced around the row.