Iterations executed serially across the regions listed below.

This usually happens with nested loops where the outer loop is serialized due to a dependency in the inner loop. In such cases the outer loop is pipelined, but it will be stalled until the inner loop is fully executed. What the initiation interval of two means in this case is that even if the inner loop finishes in one clock, the outer loop will be executed once every two clocks. In every other case, the outer loop is basically serialized. You could assume the two clocks is the "minimum" initiation interval of the outer loop.