On both rows 3 and 5 you are casting on at the beginning of a row. You have the needle with the stitches on it in your left hand, using either the knit cast on or the cable cast on (see videos under "cast on", on the purple tabs above) cast on 5sts at the beginning of row 3, or 4sts on row 5. You basically knit the stitch loop up and put it back on the left hand needle. On rows 3 and 5 after you have added the 5 or 4 stitches you then purl across the stitches you just added and then purl the rest of the row.
At the end of row 4 you also increased one stitch at the end of the knit row, so now you should have 35 if you are making the smallest size.
Your next instructions are:
Cont in stocking st, inc one st at front edge in next 7 rows, then in foll alt rows 6 (7-7-7) times … 48 (57-63-71) sts. Tie first marker at end of last row. Cont inc at front edge in foll 4th rows 6 (5-5-5) times … 54 (62-68-76) sts.
So now you increase on each of the next 7 rows when you are on the front edge of whatever it is you are making. One time it will be increased on a purl side and then on the knit side. Every row for 7 rows 35+7=42. Then you increase every other row when you are on the front edge 6 more times. So all of these will either be on the purl side or the knit side. 42+6 is 48.
Does that help?