This is a relic of the gruel which was a staple item in the daily diet of poor peasants, particularly in the winter months, during many centuries past.
Basically, porridge is a mixture of partially milled grain and water, boiled until thick. Today it is more often a breakfast food, eaten with milk or cream, and may be sweetened with sugar or honey. It is usually made with rolled oats (see Oatmeal), but rolled wheat (wheatmeal) can also be used for porridge. Buckwheat is used in Russia to make a similar kind of porridge, called kasha (see Kasha), and in those areas of Europe where maize is grown, coarsely ground maize is the preferred grain.