select attribute used to specify the XPath value where you want to get the item from. Item may be an element likes "/rootElement/subElement", attribute likes "/rootElement/subElement/@color" or grous of attributes likes "/rootElement/subElement/@*" or expression likes "/rootElement/subElement[@color = 'blue']".
It specification depends on the XSL tag that you are using.
<xsl:value-of select="..."/>
<xsl:for-each select="...">
...
</xsl:for-each>
<xsl:element select="..."/>
<xsl:apply-attributes select="..."/>
better understand to XSL, here is a good tutorial.
http://www.xfront.com/xsl.html