The Tails Array Lies

$O(n \log n)$ longest increasing subsequence on a = [3, 1, 4, 1, 5, 9, 2, 6].

vector<int> tails;
for (int x : a) {
    auto it = lower_bound(tails.begin(), tails.end(), x);
    if (it == tails.end()) tails.push_back(x);
    else *it = x;
}
// answer = tails.size()

What is tails after the last two elements, and what does it represent?