What does passo mean on knitting pattern?

The typical abbreviation is psso and it stands for "Pass the slip stitch over".

This stitch is used to decrease and is usually part of the stitch abbreviation SKP, which stands for "slide 1, knit 1, psso"

1. To do this decrease, you want to take your right needle and insert it into the stitch on your left needle and move the stitch to your right needle. So you have just slipped the stitch from one needle to the other, without knitting or purling it.
2. Now you want to knit the next stitch.
3. Then take the slipped stitch and pull it over the knit stitch. This is passing the slipped stitch over the knit stitch, which means that you have 1 less stitch than you did before.