The Poppy Appeal is an annual charity event organized by The Royal British Legion in the previous weeks before Remembrance Sunday (the second Sunday of November).
Plastic red poppies are attached to the clothing as a return to a small donation.
The Poppy Appeal first started in Britain in November of 1921, and it was inspired by the 1915's poem 'In Flanders' Fields' written by Joe McCrae:

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.
We are the dead. Short days ago
We lived, felt dawn, saw sunset glow,
Loved and were loved, and now we lie

In Flanders' fields.
Take up our quarrel with the foe;
To you from failing hands we throw
The torch; be yours to hold it high,

If ye break faith with us who die
We shall not sleep, though poppies grow
In Flanders' Fields.

The funds from the Poppy Appeal are raised to provide financial, emotional and social support to the ex-service community of the British Armed Forces.