It is a poppy, a symbol many Brits wear on their lapels in the first weeks of November. The poppy is the symbol of those that died in the great wars, and it comes from the poem "In Flanders' fields." in which the poet talks about the poppies growing between the thousands of graves of those who died in the war.
On November 11, 1918 the First World War finished, and to remember those who died in the wars, people keep silent for a minute at 11.00. It is on the eleventh day of the eleventh month and at the eleventh hour when hostilities finished. A special ceremony takes place on the Sunday closest to this date, and that was yesterday. So here's a video of the ceremony at the cenotaph in London.