I wonder what is the meaning of curly brackets{} in the regular expressions {a + ab}* and how are they different from the round brackets() in regular expressions.
If this is regarding the syntax of theoretical regular expressions, curly braces are not part of the usual definitions I have seen which only include parentheses. However, I imagine that they serve the same purpose and are probably used by analogy since every regular expression and subexpression is related to a language (a set of strings) and curly braces are usually used to denote sets. In the same way, you sometimes see people using U (union) or | (logical or) instead of + in regular expressions to mean the same thing.
In short, {a + ab}* almost surely means the same thing as (a + ab)*.