What's a two-step ppd test?

As it says: In some people with previous exposure to tb, especially remotely, an imitial TB skin test (tst) will be negative but a repeat, say 2 weeks later, can be positive. It is sort of like waking up the immune recognition of the bug. But, it is not at all clear that those who are only positive on the two-step tst are at risk for reactivation later as those with a 1-step positive tst.