Asterisks (*) are used to mark the beginning of a repeat, either a part of the patterns or a part of a row. The following is an example of a pattern repeat.
Instructions for back and front of a sweater may have an * at the start of the armhole shaping on the back. Then, when you get to the front, the instructions will state that work as given for back to *. This way, you should repeat the instructions given for the back. When an asterisk appears in instructions for rows, it may be used as a stitch repeat, as indicated in what follows.
K. I, * P. I, rep. From *to end of row. This simply means that you should repeat P. (purl 1), K. I (knit) again and again on the row until the row is finished.