Sometimes you will see the instruction in a pattern to "knit the knits and purl the purls as you see them" or K the Ks and P the Ps. All this means is that you're knitting the opposite of the row you just knit.
When looking at the back side of a row, you'll see the opposite stitch from the one you just worked. Knit stitches look like purls, while purl stitches look like knits. It's this backward quality that makes Stockinette Stitch work because the front of a knit and the back of a purl look the same.
To determine whether you should be knitting or purling a stitch when all you're told is to knit the knits and purl the purls, you can either look at the last row of knitting and do the opposite (also from end to beginning) or just look at the stitches themselves.
A knit stitch (which you would have purled on the other side) just looks like a plain piece of yarn looped over the needle while a purl stitch has a little bump at the bottom.
If you were knitting the knits and purling the purls on the knitting in the accompanying picture, for example, you would purl the first three stitches, then knit 1, purl 1, knit 1, purl 5 and knit 1.
Once you've gotten a little practice "reading" your stitches, you'll be able to knit the knits and purl the purls in a flash.