shaping neck

Hello,I am not a very experienced knitter but I find it relaxing.
I often come across instructions which confuse me.
At the moment I ma making this jumper and I got to a part where I need to shape neck.I knitted the front of the jumper and ended it with a purl row as instructed.Then it says to :
next row : knit 31 sts , turn and work on these sts for first side neck…what does it mean precisely…?
Do I knit on a side with purl stitch?and then turn it and knit …what does first side of neck mean?
Then instruction says : decrease one st at neck edge on every row until 20 sts .
continue straight until front measures same as back to shoulder ,ending at side edge.
Cast off for shoulder.
I really would appreciate some help .I f someone could shed a like on this I would be most grateful.
Thanku you.

Welcome!
You’re going to be working the part of the sweater from the lowest part of the neck to the top of the shoulder. This is the first shoulder or first side of the neck. Once this is finished you’ll work the second shoulder or side of the neck.
Knit across 31 sts, turn as you would at the end of a row, decrease one stitch and purl to the end of the row. Keep working back and forth on these sts until 20sts and the correct length.
What is the name of your pattern?