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.
That sounds more confusing than it is. All this means is that you're knitting the opposite of the row you just knit. In other words, you should knit the stitches that look like they were just knit. And you should purl the stitches that look like they were just purled.
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 (knitting one row and purling the next row) 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 either can 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 in a V shape, while a purl stitch has a little bump at the bottom. When you turn your work and are ready to start the next row look at the stitch you are about to work. If it is a V, then it is a knit stitch so you knit it. if it is a bump, then it is a purl and you purl it.
If you were knitting the knits and purling the purls on the knitting in this picture, you would purl the first three stitches, then knit 1, purl 1, knit 1, purl 5, and knit 1.
A seed stitch, for example, is made by knitting one stitch and purling one stitch on one row, and then purling the knit stitch and knitting the purl stitch on the next row.
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.