Poppies are flowers in the Papaveraceae genera. The various types have different colors and have different uses. The most famous is probably papaver somniferum from where we get opium, but there are many other ornamental varieties that are grown around the world. As a name, Poppy became especially popular after WWI in honor of the 1913 poem written by Canadian WWI veteran Lieutenant Colonel John McCrae "in Flanders Fields," which starts:
In Flanders fields the poppies blow
Between the crosses, row on row,
That mark our place; and in the sky
The larks, still bravely singing, fly
Scarce heard amid the guns below.
Largely due to this poem, the poppy has become the symbol of remembering WWI veterans. In many places, Veterans' Day (also known as Remembrance Day), the day commemorating the end of WWI, is informally called Poppy Day.