Both knit and crochet patterns often feature groups of stitches set apart by parentheses or brackets. When you encounter these groups, there should be an instruction immediately following the parentheses or brackets that will apply to those stitches.
You might have something like:
(sc, ch 3, sc) in next ch-5 sp
All this means is that in the next ch-5 sp you are going to work a sc, then ch3, and then in the same space, make another sc.
Or, you might have instead:
(k2, p2) x5
This indicates that you are to repeat all of the stitches in the parentheses, in order, five times. Written out this would be k2, p2, k2, p2, k2, p2, k2, p2, k2, p2.
There is no difference between parentheses and brackets. Generally you will see brackets inside parentheses, which just means that you have a group within the group — so for instance you might be working the bracketed group of stitches into a single stitch as you are here, while at the same time repeating the entire sequence of stitches in parentheses:
(k2tog, [k1, yo, k1] into next stitch, k2tog, p2) x3
Again, you are to work all of the instructions in the parentheses 3 times, and in this case part of that grouped repeat will be a double increase created by working a k1, a yo, and another k1 all in the same stitch.
You can even have multiples grouped inside other multiples:
(sc, ch1, sc, [sk2, ch3, sc] x 3, ch1) x2
Here you are working everything in the parentheses twice, and that grouped repeat includes working everything in the brackets three times. I’ll write this one out for you, because it can get confusing:
sc, ch1, sc, sk2, ch3, sc, sk2, ch3, sc, sk2, ch3, sc, ch1, sc, ch1, sc, sk2, ch3, sc, sk2, ch3, sc, sk2, ch3, sc, ch1
I think you can see why we use the shorthand of grouping stitches — trying to follow along those long lines of instruction without getting lost or accidentally repeating a stitch can be very difficult! Shortening these instructions down helps to clarify them both as you read and as you interpret the structure of the pattern.
If you’re getting confused by a grouping or aren’t sure how the stitches are going to add up, go ahead and write it out! This is another one of those conventions in knit and crochet patterns that gets easier to interpret on the fly with practice, so don’t feel like you’re alone if you’re not getting it right away. And don’t forget: stitch markers are your friend. Having grouped stitches like this makes it easy to tell where a stitch marker might be handy — put one between each group, using different colors for the internal groupings if you want to mark those as well. Then if you do run into a problem you can very quickly isolate it, fix it, and get on with your project.