It signifies a function. Sometimes you need to put stuff inside the parenthesis, sometimes not, depending on function. But functions always need them to identify as functions instead of variables.
I will write a more detailed example in a custom mql code block, but you will have to troubleshoot it from there (using the Print() function for example)
datetime time = TimeCurrent();
string timestring = TimeToString(time);
int day = (int) StringSubstr(timestring, 8, 10);
Print(day);
Print("Today is " + day + "th day of the month");
