Hm, well if anyone's really wondering, this is a knitting stitch. Since I don't listen to Phish, I don't know why they chose to reference knitting....*
Slip Stitch and Pass (Slipped Stitch Over next stitch - often abbreviated ''Slip n and PnSSO'') is a way of decreasing your total number of stitches by the number slipped (usually 1 or 2), with the decrease appearing to slant to the left: \\ .
This stitch is done by moving without knitting (ie. slipping) the stitch(es) you wish to decrease onto the receiving needle (the right hand needle, unless you knit lefty). Then, the next stitch is knit as usual. The slipped stitch(es) is then pulled over the newly knit stitch so that it rests on top of the stitch next to it. The result is that you have fewer stitches on your needle and the row you just knit off of will have overlapping stitches.
This is identical to Knitting 2 stitches Together, except that K2tog results in a right slanting decrease: // . The slant depends upon which stitch is on top of the overlapping stitches. Slipping permits the knitter to change which stitch will be on top. When decreasing 2 stitches working right to left (as most knitters do), if the 1st stitch is on top the decrease will slant left (\\). If the 2nd stitch is on top, it will slant right (//). A centered double decrease can be achieved by knitting 3 together with the center stitch on top. This can be done by slipping 2 stitches knitwise, knitting the 3rd stitch and then passing the two slipped stitches over the newly knitted stitch. Slipping knitwise refers to slipping the two stitches as if they are going to be knit together. This twists them so that the 2nd stitch is on top.
The direction in which the decrease slants is important in many textured and lace stitch patterns. Attention to fine detail results in leaves and other designs which look layered and defined instead of pixelated. Think about ascii art and the limitations of trying to represent a picture with only v-shaped blocks in a grid, and you'll see why being able to create a slant is important. Many times, these slanted and centered decreases are paired with corresponding numbers of increases so that the total number of stitches does not change. In this case, they are used solely for their slanted appearance.
I should mention that these instructions are not going to give the same result for each and every knitter because of variations in basic knitting technique. These instructions are fairly standard, but not universal. For example, when I knit, I get a left slanting decrease (\\) by k2tog because of the way my stitches are oriented on the needle. In order for me to get a right slanting decrease (//), I have to SSK (Slip, Slip, and Knit the slipped stitches together).
The best thing I can recommend if you come across any of these instructions in a pattern is to figure out why the pattern requires it. If it's necessary, and not just a randomly chosen decrease, do whatever it takes to achieve the correct result and don't worry if it isn't what the instructions say. Also, consider using slanted decreases where they aren't requested, in order to give that garment a little extra finesse. Either way, your sweater, scarf, shawl, socks, mittens, muff, hat, stole, shrug, snood, sampler, afghan, throw, etc. will be all the better for it.
interrobang says re Slip Stitch and Pass: I don't know why Phish chose this title either, but the cover of the album pictures a guy running from a giant ball of yarn. -(For some reason, this makes me laugh....)