Brackets [ ] and also ( ), which you have here, indicate things that are to be repeated.
Row 1 - K1 YO (K2tog, YO) across to 1st marker, ending with (k1, YO), SM, K1, SM, YO, K1, YO (then SSK, YO) across remainder of row, ending with K1.
You will begin row 1 with the K1 and then do the YO. Then you will repeat (K2tog, YO) all the way across until you get to the first marker. So it will go K1, YO, K2tog, YO, K2tog, YO, K2tog, YO etc. until you get to the marker.
SM means slip the marker, so do that, then you K1 and slip the next marker. Then you do the YO, K1, YO and then repeat what is inside the ( ) the rest of the way across the row. So you do the SSK, YO, SSK, YO, SSK, YO until you come to the last stitch, and you knit that one.