What does "sc in the next sc" mean?

The previous answer is great. Since you mention the chain, I am wondering about your pattern. It is possible that the pattern is a repeat and the designer just started in, uming the crocheter would realize to make the switch between stitches and chains. Whether you are working in the foundation chain or the succeeding rows, I think your designer means for you to alternate single crochet and double crochet, working one in each stitch. Just as it says, first work a single crochet directly in the next stitch, going through both loops. Then in the next stitch after that one, work a double crochet, going through both loops. I am just guessing, but I think on the next row you will do the same, but you will be working the dc into the sc and the sc into the dc. As I say, without knowing the pattern, that is just a guess. Good luck.