The Horse chestnut (Aesculus hippocastanum)
is not native to the UK but was brought to Britain in 1616 from south east Europe. In the spring they have characteristic "candles" of white flowers, which grow up to 20cm tall. The flowers are hermaphrodite (made up of both male and female parts) and once they have been pollinated by insects, the colour in its middle changes from yellow to red as a warning to bees not to bother with it anymore! Once pollinated they produce conkers. The most famous use of these is in the game of conkers. The first record of the game is from the Isle of Wight in 1848. No one knows where the name conker comes from, but some people think it could be from the French word for to ‘knock out’ (from when it used to be played with snail shell) or from similar earlier games called conquerors.
For more info see https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/british-trees/a-z-of-british-trees/horse-chestnut/