Before starting this lesson, be sure that you fully understand
Learning to knit and purl in the same row means learning how to transfer the yarn between the stitches when changing from knit to purl or purl to knit. When knitting the yarn has to be to to the rear, and when purling the yarn has to be in the front.
Seed Stitch (as it is called in American Knitting or Moss Stitch as The British call it) is a 1x1 rib pattern. The need to transfer the yarn before every stitch makes this a very difficult rib pattern. Learning this first makes all other rib patterns easy.
Many patterns will do this on an even number of stitches so that the first stitch of every row is a knit stitch. When the knitter is presented with an odd number of stitches, every other row starts with a purl stitch. Knowing what row you are knitting becomes necessay. Knowing how to "read your knitting" becomes a vital skill.
Seed stitch produces a wonderful pebbly fabric that is a bit firmer than stockinette. My knitting friends love the look of this fabric but moan when they have to knit it because of the constant back and forth yarn transfers which does slow the knitting time down a bit.