The worldwide flood is a flood that covered the entire earth in Noah's time. God sent it because he was angry with the peoples' sins, so he destroyed all of mankind as a punishment, except for Noah and his family by instructing him how to build the ark. Every animal was destroyed except for two of every unclean animal and 7 pairs of clean animals.
When it was all over, God sent a rainbow as a promise to mankind that another worldwide flood would never come again. Many floods have come since, some large, some small, but none have ever covered the entire earth. That was the only flood that has ever covered the whole earth.