They tell you what operations to do first. You work from the innermost to the outermost.
For example, 10-(7-3) is 10-4=6 because we have to take 3 from 7 first, making 4, then we subtract the result from 10. Parentheses or the P in PEMDAS which tells you the order you give to various operations in a sum. Brackets are the same as parentheses and you can use the acronym BODMAS like PEMDAS.