I’m knitting Carina Spencers Vespertine Cowl. Theres a stitch descripted as sl-dw= slip the next stitch purlwise, dropping the second wrap. This makes no sense to me 
so I’m hesitant to start. Is there an alternate description more commonly used that has a video to shows what this looks like somewhere?
Welcome to KH!
It sounds like there is a stitch on the previous row that has a double wrap. This is to elongate a stitch or to give more yarn in the stitch possibly for the cable. Check the previous row for that double wrap.
If that’s so then you would just drop one of the wraps as you slip the stitch from left needle to right.