Stockinette stitch is simply you knit the first round, and then purl the second and repeat. This puts all the knit stitches on one side and the purls on the opposite.

There are 2 basic knitting stitches: stockinette stitch and garter stitch. The stockinette stitch is where you knit one side and purl the other which gives you a smooth side and a bumpy side. Garter stitch is where you knit every row which gives you a bumpy look on both sides.

stockinette or stocking stitch means knit one row, purl one row. So four rows are knit one row, purl one row, knit one row, purl one row.

There are many kinds of stitches in knitting. There is normal stitch, purl stitch, garter stitch, stockinette stitch, ribbing stitch, seed stitch and lots more. Loads of stitches are actually made up of other stitches merged, chopped and changed.

Assuming you are doing stockinette stitch (knit the front side and purl the back side), your knitting is curling because that's precisely what stockinette stitch does. To make it stop, put a band of garter stitch (knit the front side AND the back side) or ribbing around the piece and that will make it lie flat.