Nothing to mess up buddy!!
If we have a positive valued functions f(n) and g(n) takes a positive valued argument n then ?(g(n)) defined as {f(n):there exist constants c1,c2 and n1 for all n>=n1}
where c1 g(n)<=f(n)<=c2 g(n)
c1=5 and c2=8 and n1=1
Among all the notations ,? notation gives the best intuition about the rate of growth of function because it gives us a tight bound unlike big-oh and big -omega which gives the upper and lower bounds respectively.
? tells us that g(n) is as close as f(n),rate of growth of g(n) is as close to the rate of growth of f(n) as possible.