Poppies have become associated with 'Remembrance for the Fallen' Day in November. The reason for this is because red corn poppies grown grow naturally in the wild in Western Europe. In 1914-1918 the fields the fields of Northern France ie: Flanders, Ypres were destroyed by the destruction of the war. The red poppy was the only flower to bloom on the battlefields. The blood red colour of the flower represents the blood of the soldiers who died. Also, I think that there was a famous war poem written after the First World War called 'In Flanders Fields'. That poem also mentions the poppy. During the weeks leading up to Remembrance Day, it is popular for British people to wear a red poppy on their clothes to show respect for the soldiers who died.
Hope this helps!