How to Slip Stitches
In knitting-speak, to slip a stitch (abbreviated sl st) means to move a stitch from the LH needle to the RH needle ("slip" it) without knitting or purling it and without changing its orientation (that is, without twisting it).
To slip a stitch, insert the RH needle purlwise (as if you were going to purl) into the first stitch on the LH needle and slip it off the LH needle onto the RH needle.
Unless your instructions specifically tell you to slip a stitch knitwise, always slip a stitch as if you were going to purl it. Here’s a look at stitches being slipped both purlwise (a.) and knitwise (b.).
You frequently run across slipped stitches in methods for decreasing stitches — when you want to reduce the number of stitches you have on your needle. They also form the basis of a family of stitch patterns. Like garter stitch, slip-stitch patterns are stable and lie flat — and they’re a breeze to knit.