A simnel cake is a spiced fruit cake with both a top and a middle layer of marzipan. These cakes were originally made for mothering Sunday and sometimes still are, but nowadays we associate them more with Easter. There are many different recipes for this cake; in one version of the story, it was created by a man called Sim and his wife, Nell. They mixed a cake and couldn't agree whether to boil it or bake it, so finally they agreed to do both - and some versions of the simnel cake are boiled and then baked, but we don't know whether this story is true.
There is also a tradition that the cake should be decorated with eleven marzipan balls - one for each of the 12 apostles apart from Judas.
If you would like to try baking a simnel cake, there is a good recipe here though it has been slightly simplified.