Remembrance Day falls on the 11th of November each year and it marks the anniversary of the armistice which ended the First World War (WW1) that ran from 1914 to 1918. 
Each year Australians and other allied countries observe a one-minute silence at the 11th Hour of the 11th Day of the 11th Month in memory of all of those who died or suffered in wars and armed conflicts.
When hostilities ended in 1918, it was known as Armistice Day, to honor sacrifices made by all who fought and lost their lives during the First World War (WW1). 
At the conclusion of the Second World War, the Australian and British Government's renamed Armistice Day to Remembrance Day, to remember all those who had died or suffered at war.
In 1918, Edward Honey, an Australian journalist first suggested the ritual of observing one minute of silence and today continues to be universally practiced on Remembrance Day each year around the world.
Red Poppies are also associated with Remembrance Day as they were among the first plants to bloom in the devastated battlefields of northern France and Belgium during the First World War (WW1). Soldiers often associated the vivid red of the poppy with the blood of their comrades that soaked the ground, which has made the poppy symbolic of the bloodshed suffered in trench warfare.
More information about the origins of Remembrance Day can be found here.