Dear Silke,
a shortened row is indeed a “shorter” row. You knit the specified number of stitches, then turn the work without having knitted the rest of the row or round.
This means that after you’ve turned the work for the first time, the wrong side of the piece is facing you, which is why you knit back in purl. When you reach the point with the yarn over, knit the yarn over and the next stitch together, then continue knitting until you’ve reached the number of stitches specified in the pattern. Then turn the work over again without knitting the rest of the row or round.
I hope this helps a little.
Best regards, Steffi
P.S. I’m attaching a picture to show what actually happens with the shortened rows. The orange section is knitted with shortened rows (even though I use a different technique for the turning stitches).