It sounds as if you will be doing "centered increases". Something has been left out of the instructions - try this: "k to 1 st before marker; yo, k1, slip marker, k1, yo". Your increases will be balanced on either side of the marker.
If this solution throws your row off, please repost and give us your stitch count and the instructions for the entire row so we can be of more help to you.

Ann B, what you said makes sense, but to be sure, here are the instructions. This is a pattern for an American Girl Doll sweater.

Using size 4 needles, cast on 49 sts. K 3 rows.
Change to size 6 needles, K4, place marker, P7, place marker, P13, place marker, P7, K4. Begin pattern as follows:

Row 1 K4(front band), *K to 1 st before marker, yo, K1; K1, yo*. Repeat across, end K to last 4 sts, K4.

Row 2 K4, P to last 4 sts, K4.

Repeat rows 1 and 2 until there are 21 sts on each front, 27 sts on each sleeve, and 33 sts on back. End by working a wrong-side row.

I can figure out the rest. Does the semicolon in that line have any special meaning?

It makes sense to me (but I've been told I'm senseless). Think of the semicolon as your marker. As Anne said, one stitch before the marker you yarn over and knit one. Then slip your marker. Then knit one and yarn over. Knit to within one stitch of the next marker and repeat. You are thinking of your marker as the end of that itty bitty part of the instructions, and it is not the end. Does that help?

Yes it makes sense. Thank you to both Anne and Donna.