The people who lived at the time of Noah were not honest - they stole, robbed, told lies - you name it, they did it. They were wicked and did not follow in G‑d’s ways. The only righteous person in the whole generation was Noah. G‑d tells Noah that He is planning to destroy the entire world by bringing a great flood.

G‑d tells Noah to build an ark - a teivah - where he and his family as well as any people who will repent could escape from the flood. It takes Noah 120 years to build the ark because G‑d wants to give the people a chance to repent. Whenever people pass by and ask him what he is doing, he says: "I am building an ark, because G‑d will destroy the world if you do not repent. Repent, there is still a chance! Start behaving honestly and become good people!" But the people laugh at Noah and don't take him seriously.