Hand Knit Baby Sweaters Knitting Question For The Experienced (and Unexperieanced If You Have The Answer) Knitter?

Knitting Question for the experienced (and unexperieanced if you have the answer) knitter? - hand knit baby sweaters

Hello! I was knitting to see, four years ago, but nothing harder than scarves, laundry and baby sometimes. I am trying my hand on a sweater, and I met some of the wording of weaving does not understand. What he says is the "point of STS annually on the left side of the needle." What does this mean and how can I do? Thank you in advance for your response and I truly appreciate your help!