SSK is an alternative way of working the old "slip 1, K1, pass slipped st over", which is a leftward slanting decrease.

Slip next st from left hand needle to right hand needle knitways. Slip next st the same way. Return both sts to left hand needle in their new twisted positions and k2tog through the backs of the loops.

Here is a video I found on google video that shows how to work SSK. Heather also has other excellent videos listed on her blog at http://www.lectio.ca/