Help with double stitch please


I have got a lovely vintage knitting pattern from I think the 1940s. Can anyone explain the term “double stitch” which appears in the instructions. I believe you knit the second stitch on the left hand needle to create a pattern, but I am not sure if this means then knit the first stitch above as well. Any nice clear instructions would be really appreciated. Many thanks.

Insert needle from front to back into the stitch below next stitch,which will be formed of double strands, and draw through a loop; then knit the next stitch and pass the loop over the stitch just knitted

Thanks, I will experiment!