If something is 'as dead as a dodo', it is no longer important or popular. This phrase is often used to talk about ideas or trends.
In the past people used to watch and record videos on VHS, but now that format is as dead as a dodo.
The idea that smoking is good for your health is now as dead as a dodo.
A 'dead duck' is something that is not successful.
Frank's coffee shop business was a dead duck sadly. His coffee was just awful, and the shop was in a bad location.
The flightless dodo was first discovered on the Indian Ocean island of Mauritius in the late 16th Century. Within a few decades they were extinct. Before the arrival of man, the dodo is thought to have evolved to such a large size because it had no natural predators and plentiful food.