I need to know what [U]ssk and (ssk) [/U]mean? It is in a pattern I would like to try.
Check out the Glossary tab at the top of the page. All the abbreviations and many with video to go with them.
What Ingrid said 
ssk means slip, slip, knit - I slip 1 stitch knitwise, slip 1 stitch knitwise, and then knit through the back of both stitches.
It’s a decrease stitch…hope that helps a bit.