Yrn is yarn round the needle, a yarn over usually after purl sts. For this romper pattern it’s a plain old yarn over between knit sts in order to increase the stitch count and make the eyelets that you see at the raglan lines. Just bring the yarn strand to the front between the needles then over the right needle to the back. That’s it.
The yarn overs for the buttonholes
K2, yrn, k2tog, (makes a buttonhole)
are noted in the pattern on rows 3,15,27 and then in the: *NOTE: Buttonholes on rows 39 -51-63-75-87