苇 is a bound form (b.f.) meaning 'reed'. It cannot stand alone as a word; it always appears in compounds, most commonly 芦苇 (lúwěi, 'reed'). You'll also see it in 苇子 (wěizi, 'reed stalk') and 苇塘 (wěitáng, 'reed pond'). When you want to talk about reeds in general, use 芦苇 rather than 苇 by itself.