The stem is the most forward part of a boat or ship's bow[1] and is an extension of the keel itself.
[1] (For example, "The hull is single decked and characterized by a plumb stem, full bows, straight keel, moderate deadrise, and an easy turn of bilge.
"[3]) Because the stem is very sturdy, the top end of it may have something attached, either ornamental or functional in nature.
On smaller vessels, this might be a simple wood carving (ornamental) or cleat (functional).
On large wooden ships, figureheads can be attached to the upper end of the stem.