These surveying tools are used to mark trails or other feature needing easy identification. They are sometimes called "trail markers" or alternately "trail whiskers". They come in an assortment of colors and are hammered into the ground with a large spike, leaving a 6" high bright grass-like sprout of color that is easily seen and followed.
They are often used by hunters to mark a route, and have many other uses in urban society including landscaping, utilities, etc.
They are quite durable and generally last several years outdoors before deteriorating. Athletic facilities often use them to mark turf fields for painting athletic lines/boundaries as lawn mowers do not damage them.