What is asymptotically tight bound?

What this notation T(n)=Θ(n2) means

For a given function g(n), we denote by Θ(g(n)) the set of functions 

Θ(g(n)) = {f(n): there exist positive constants c1, c2 , and n0 such that 0≤c1g(n)≤f(n)≤c2g(n) for all n≥n0}.

We could write “f(n)∈Θ(g(n))” to indicate that f(n) is a member of Θ(g(n)).
Instead, we will usually write “f(n)=Θ(g(n))” to express the same notion. The abuse may at first appear confusing, but it has advantages.