Slip one stitch knitwise (Sl1K), which means to move on stitch from the left needle to the right needle without actually knitting it. You will insert your right needle into the stitch as though you were going to knit it (so, from the front left of the stitch if that makes sense) and move it to the right needle.
You will then knit together the next two stitches on the left needle.
Finally, you will take that slipped stitch (which is now the second stitch from the tip on the right needle) and lift it over to the left and right off the needle.
All of these steps together are a double decrease (two stitches decreased).
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