beginning neck shaping

it says to put center stitches on holder and remaining stitches on another holder. decrease at neck edge every alternate row. If stitches are on two holders I’m not sure what every alternate row wold be…Help**

Welcome to KnittingHelp!
Usually you’ll knit across a given number of sts for the left shoulder (as you wear the sweater) and then place the remaining sts on a holder or holders for the center neck and right shoulder. Then you turn and work back to the armhole edge on the left shoulder sts.
For the next row, a right side row (RS) you start at the armhole edge and work to the neck edge. You could decrease toward the end of this row Don’t decrease on the next row (the WS row). If you keep decreasing on the RS rows, that’ll be every other row or every alternate row.