It is an increase that turns one stitch into two, so each row you are increasing the amount of stitches in that row. You knit like usual into the front leg of the stitch, then WITHOUT SLIDING THE STITCH OFF THE LEFT NEEDLE YET, then knit into the back leg of that same stitch, THEN slide it off the left needle. It might take a bit of finagling and might be a bit tight to get that stitch done into the back leg, but it CAN be done.
Row 1 β 1 + 1 = 2 when you are done with the row
Row 2 β 2 + 1 = 3 β¦
Row 3 β 3 + 1 = 4 β¦
Row 4 β 4 + 1 = 5 β¦
Row 5 β 5 + 1 = 6 β¦
Row 6 β 6 + 1 = 7 β¦ when you have completed row 6