Knitting pattern help?

Almost. It's 100 stitches, not 100 rows. So if you do the increases every 5 rows until your stitch count is 100. Supposing you start with 90 sts.

row 1: increase row (92 sts)

row 6: increase row (94 sts)

row 11: increase row (96 sts)

row 16: increase row (98 sts)

row 21: increase row (100 sts)

You've reached 100 sts, so this would be the point where you tie a scrap of yarn on the first stitch and another on the last stitch, to remind you where the sleeves will go when you get around to assembly. Note that this marker goes on the fabric, on an individual stitch. It is row marker, not a stitch marker. Stitch markers go on the needle between stitches and are slipped each row so they travel with the needle. Row markers are inserted into one individual stitch and remain in that stitch even when the needles move on to new rows.

It means increase 1 stitch at the beginning and end of the first row.

Then do stockinette for 4 rows and on the 5th increase at6 the beginning and end again.

You do stockinette stitch for 4 more rows and then do the increases again. You continue doing this until you've done it 26 times and then you should have 100 stitches.

That 100 refers to the number of stitches you will have not the number of rows, so your interpretation is wrong. There should be a period after (26) times and then it gives you the number of stitches you should have