Does anybody know where the component 'messages' does come from? Like 'messsages.down' or 'messages.up'

In an example like:

<s:link styleClass="columnHeader" value="idBanco #{bancosList.order=='idBanco asc' ? 
messages.down: ( bancosList.order=='idBanco desc' ? messages.up : '' )}">
<f:param name="order" value="#{bancosList.order=='idBanco asc' ? 'idBanco desc' : 'idBanco asc'}" /> </s:link>