Need help with pattern

Sorry guys i’m new to patterns and this has me stumped.
So I’m knitting a baby smock and i’ve started on the back, i’ve done the divide for the back opening so have one bit longer than the other. Now the next instructions have me stumped. leave rem sts on a stitch holder. I’m gathering that that is the longer bits (they are on the end of the needles). join yarn to rem sts, cast on 5 st for underlap. What??? I’m confused. then it continues on to what looks like the other side of the divide.
What is an underlap, and how do i join? Do I cut the yarn that i’m working with? Sorry guys.

To join yarn, you just take an end and begin using it, leaving a tail so you can weave in later. The underlap is like a button flap I bet. So put the shoulder sts on a holder, take another end of yarn and go back to the other side, CO 5 sts (use the knit or cable CO) and work that side. You can cut the yarn, leave a tail for weaving in.

Thankyou so much, yes it is the back of the button hole area. I had a feeling that that is what it was ment to mean. IDK maybe baby mush brain lol.