What it does is that it restores state of rendering a form.
For example I had this form:
<s:validateAll>
<h:message for="name"/>
<h:outputLabel for="name" value="Grupi nimi"/>
<h:inputText id="name" value="#{category.name}" styleClass="field"/>


<h:message for="head"/>
<h:outputLabel for="head" value="Tüvikategooria"/>
<h:outputText id="head" value="#{category.head.name}" styleClass="field"/>


<h:message for="showed"/>
<h:outputLabel for="showed" value="Avalik"/>
<h:selectOneRadio id="showed" value="#{category.showed}" styleClass="field">
<f:selectItem itemValue="true" itemLabel="Jah"/>
<f:selectItem itemValue="false" itemLabel="Ei"/>
</h:selectOneRadio>


<s:link view="/haldus/mcategory.xhtml" id="submit" value="Loo grupp" action="#{cateditor.create}"
buttonClass="button" linkStyle="button"/>
</s:validateAll>
and after submiting it, it restored category, so the name became null.