That whole ‘knit the knits and purl the purls’ really got me at first too. I had NO CLUE what that meant .
Stockinette stitch, as you are learning, is knit one row, purl one row. The side that looks like rows of V’s is your knit side. If that side is facing you as you hold your needle full of stitches in your left hand with your working yarn nearest the tip of your needle, you will be knitting. If the loopy, bumpy, purl side is facing, you will be working a purl row next. Does that help??
Now as for the knit the knits and purl the purls…I hope this doesn’t confuse you more but it finally clicked for me when someone on this site said that when you knit a stitch on the right side, it forms a purl stitch on the wrong side. So therefore when you are doing stockinette and are working a whole row of knit stitches, you are forming a whole row of purl stitches on the reverse side which will then be facing for your next row. So when you start the next row, you will be purling the purl stitches that were formed during the previous row.
Sheesh…that was a mouthful and I really hope I didn’t confuse you.
Bottom line with stockinette stitch…smooth side with V’s is your knit side, bumpy looped side is your purl side.