You're going to work on one shoulder of the back at a time. For this shoulder, K16, turn as you would at the end of a row. Leave the other sts on the needle or on a holder. You'll work with them later. Work back on the 16sts, dec 1 st at the beginning of the row and then on the next row at the end of the row and on the 3rd row, at the beginning of the row. All the decreases are at the neck edge rather than the armhole edge. Now you have 13sts on the needle and you work 4 rows without decreases.
You're making a gradual slope to the neck shaping. These instructions are actually easier to do with the stitches on the needles although it's always a good idea to read ahead and see what's coming up.