<xsl:template match="z:row">
<data>
<xsl:variable name="data" select="."/>
<xsl:for-each select="$outputdef/root/column">
<xsl:element name="{@tagname}">
<xsl:value-of select="$data/@*[name()=current()/@dbcolumn]"/>
</xsl:element>
</xsl:for-each>
</data>
</xsl:template>

I understand the use of the xsl:variable,

but the rest of the expression
is still baffling to me:

select="$data/@*[name()=current()/@dbcolumn]"