You are going to knit 6, then work A B A B A B A B A, then knit 6 again.
In row 1 of chart A, you are doing YO, k1, YO. This “uses up” one stitch to do, but also creates 2 sts, leaving you with 3. (Remember a yarnover is just that – moving your yarn over/around the needle and nothing else.) So every time you see A in the instructions for this row, you do YO, k1, YO.
In row 1 of chart B, you are doing k5, k2tog, YO, k6. This uses up 13 stitches and yields 13 sts. (Start with 13, decrease 1, increase 1, end up with 13 again.) So every time you see B in this row, you are going to repeat that sequence.
Hope that helps. Please ask for clarification if I haven’t explained it enough or answered your question.
Then you do row 2 (mostly purls so the author has written what to do for you so you don’t have to bother looking at the charts). That’s a wrong-side row.
Then you move onto row 3 of the instructions, which tells you where to do row 3 in the charts.
Basically you are knitting edge stitches, then from the charts, repeating and alternating A and B, then edge stitches again.