What does a flat section in a burn down chart mean (for example the section between days 3...

What does a flat section in a burn down chart mean (for example the section between days 3 and 6 in Figure)?

Figure Assessing progress trend with a burn down chart. An increase corresponds to an upward revision of estimated work (new work, tasks initially underestimated). A decrease corresponds to a downward estimate of remaining work (work completed, obsolete tasks taken out of the plan).