Hello what does s2tog p1 p2sso mean in a pattern please thankmu

Have you looked for a key of abbreviations on your pattern, maybe at the beginning?

It means slip two stitches together (but it does not say here if this is slip knit wise or purl wise so I suggest looking for a key or notes in the pattern) so you put your needle in as though to work the 2 stitches together but instead of working them just slip them onto the right needle.
Then purl the next stitch.
Then pass the two slipped stitches over the worked stitch and off the needle, leaving the worked purl on the needle.

Hope this helps, if it’s not clear do say, there is likely a video tutorial that will help.

Welcome to KH!
Good idea from Creations to check on abbreviations. Since this is a decrease, slip 2 knitwise as though then were one stitch.