This artifact is “smoothing board” or a “mangle board.” Its function is similar to that of an iron, that is, to smooth out the wrinkles in a piece of fabric. But they weren’t used in the same way as irons. Instead of just pushing down on the fabric on a flat surface, a mangle board was used in conjunction with a roller, a large cylinder of wood not unlike a rolling pin.
This also sometimes help to squeeze out moisture. They eventually became obsolete with the popularity of irons and the mechanization of laundry in general.
Many more intricate examples can be seen in the preview of this book “Mangleboards of Northern Europe” here. (or they can be seen in a google image search for mangle boards). They really are worth a look! And so beautiful for such a functional and useful tool.