Need help please with a vintage pattern..

Hi, I’m new here and would appreciate some help with a vintage pattern, the instructions are different to what I’ve experienced before, I’m quite an experienced knitter but this one has me stumped!

This is the item

I’m starting the romper at the back from the waist down, i’ve done the rib rows and the next instructions are : Cont in st st across central 21 sts ( I have 69 in total ) working extra sts from each end on every alt row as folls: 7st (twice), 10 sts (once)

what does the bit in bold mean?

thanks in advance

Welcome to the forum!
Work across the center 21 sts and then work across 7 more sts, turn and work back across the 28sts you just knit plus an additional 7sts. Repeat these steps once more and then repeat the same steps but work across 10sts more on each side. This should use up all the sts so that you’ll be working the full 69sts on the last row.
Darling patterns.

You’re a star! thanks very much for the speedy reply.