A slipped stitch edge is also known as a slip stitch edge or chain edge. Instead of "edge", you could also say "selvedge".
Why use a slipped stitch edge? It makes an even, non-bumpy edge that can make items like scarves look a bit more tailored. Another benefit is that it makes joining afghan blocks easier, as the edge stitches are well-defined.
How do you do a slipped stitch edge? There are three main ways to do it. The first two look the same, and the third is a bit different. It's a matter of preference.
* Slip the first stitch purlwise & knit the last stitch
* Slip the first stitch knitwise & purl the last stitch
* Slip the first stitch purlwise & knit the last stitch through the back loop (ktbl)
[Alternatively, some people prefer to slip the last stitch of a row & knit/purl the first. Again, a matter of preference.]
TECHknitting has good explanation of that last one, complete with diagrams (I love diagrams), and the video up top shows them all in action.
Happy slipping. :)