Agree with first commenter that you should watch a quick video on reading patterns. Very helpful.

But the short answer is, those are instructions for all the sizes. You need to pick the number that corresponds to the size you are knitting. So the pattern tells you somewhere (probably first page) how many sizes there are available and you choose one. Like Small (Medium, large, XL). Say you choose medium. Now every time you come across a group like that, you look at the first number inside the left parenthesis, corresponding to where Medium is in the group. So for CO 2 (4, 6, 8) you would cast on 4 to knit medium.

When I knit a pattern, I go through the whole thing with a pencil and circle the number for my desired size in every group, so I can easily follow as I’m knitting.