An anvil is a metal surface upon which smithing work can be undertaken. They typically are used with metals that have been heated in a forge although this is not necessarily the case. This need not be the anvil we are used to seeing in movies and many medieval illustrations show much simpler pieces.
Modern anvils are very expensive, and unfortunately, frequently too soft to be much use. Low-quality anvils are sometimes called ASOs ("Anvil-Shaped Objects"). As a result, antique anvils are sought-after items.
Alternatively, one can use a short length of railway rail or of structural steel I-beam with minor modifications, for occasional hobby projects.