slip, slip, knit
An SSK simply stands for slip, slip, knit.
It’s a really common decrease stitch that creates a left leaning stitch.
To do this slip the first stitch on your left needle, as if to knit, onto your right needle, and do the same with the next stitch too. Now insert your left needle into the front loops of these 2 stitches, wrap your yarn around your right needle and knit these 2 stitches together.