Hi and welcome!
All the numbers in and out of brackets refer to different sizes. So for the smallest size, the instructions become:
Rib 10, rib2tog, (rib 11, rib2tog) 3 times, rib 10
57 sts total at the end of the row
Circle the size you're making and just pay attention to those instructions. The initial "rib 10" means to work 10sts in k1p1 rib as you have been doing, then work 2sts together. The instructions in parentheses are repeated 3 times and then you finish with more k1p1 rib.
What size are you making? Can you give us a link to the pattern?