The beam is the width of a canoe at it's widest point. The interesting thing about a canoe, is it's widest point may not be at the gunwales but lower on the canoe. A canoe has what is called tumblehome which is a curve of the hull. This curve is necessary to add strength to the hull, increase displacement, and increase secondary stability. Generally, the wider the beam the more stable the canoe will be.
Shorter canoes can have very wide beams in relation to their length.
Canoe manufacturers will typically provide three beam figures. This last statistic is just as important as the other two because it will dictate the performance of your canoe. The narrower the beam at the waterline, the faster the canoe will perform as it has less resistance moving through the water. For whitewater and casual canoeing, a narrow beam below the water line is ideal. For backcountry trips a wider beam will provide for more stability.