If a pattern asks you to slip a stitch, there are two different ways that you might do that. The first one is to slip as if to knit and the second is to slip as if to purl. It's very easy. If you are going to knit a stitch, you would be inserting your right needle into the left side of the stitch. So to slip a stitch as if to knit means to slip it that way. So you don't knit it at all, you just pass it over to the right needle.
If you were going to purl a stitch, you would be inserting your right needle into the right side of the loop. So to slip a stitch as if to purl, you're going to slip it that way. It's a small thing, but it really does make a difference in how the stitches are mounted on the needle. So here's a slip as if to knit. Here's a slip as if to purl. Slip as if to knit comes from the left side. Slip as if to purl comes from the right side. That's how to slip a stitch.