Hi and welcome to Knitting Help. Do exactly as it says:
k2tog, but don't slip the stitches off the needle, leave both stitches on the left needle and knit the first stitch then you will slip the stitches. after that, k1.
By slip the stitches I am assuming it means to move them off the left needle.
A link to your pattern would be helpful.
You can't knit the left sock first. Ever.