A Slip Stitch is a stitch that is not worked. It is just moved to the right needle

Beginners are often instructed to slip the first stitch of every row; this helps tighten the edges and the result is a nicer looking fabric for a beginner.
The second picture shows a garter stitch sample with the first stitch on the right side having been slipped. That right half of the sample shows orderly rows of garter stitch starting to develop.

Although I am an experienced knitter, paracord is a stiff fiber with which to knit. As I gained experience with that fiber, the top left edge starts to show a nicer edge as well as neater rows across the fabric. This is very typical of beginners.
While stockinette stitch is covered in the next group of lessons, it is easier to see what is happening when the first stitch is slipped on stockinete fabric. Again, the right edge has the first edge stitch slipped while the left edge stitch is worked.
The two green lines enclose two rows of knitting in this picture. The elongated edge stitch on the right is the typical appearance of a slipped edge stitch. The left edge is the typical appearnce of working every edge stitch every row.

This same slipping of the first stitch of every row makes seaming faster as there are only half the number of stitches in the seam. Sock gussets make use of the slipped stitch as those enlarged edge stitches make it easier to see when picking up stitches along an edge with thinner yarns!
Slip stitch can also be done in the middle of the fabric and has wonderful design possibilities. The slip stitch covers two or more rows. Pairing stripes with slip stitch results in complicated looking blocks. Multi color yarns turn into focal points with this technique.