A bulrush is a very tall plant that grows in wetlands. Another name for a bulrush is a cattail.
If you hike near a marsh, you may see bulrushes poking up above the other grasses growing there. A bulrush tends to have a long, rounded seed head at its very top, and wide, strong leaves that can be used for weaving. The noun bulrush combines rush, "plant growing in marshy ground," with bul or bull, most likely used in the sense of "very large or coarse," as in the word bullfrog.