Check out the footnotes after the story.

Footnote #3: The brick oven in a Russian peasant's hut is usually built so as to leave a flat top, large enough to lie on, for those who want to sleep in a warm place.

The Russian peasant built their brick ovens with a flat top large enough to lie on. This gave them a warm place to sleep or lie down.

It is explained in the footnotes on the linked story.