xml xsl xpath

Hi,
I would like to know about XSL, XPATH, XML.
I see in my project code like belw

<xsl:call-template>

<xsl:with-param name="VALUE" select="lll"/>


<xsl:if test="$ABC_CD = 'AA'">
                        <xsl:call-template name="MapAmtWithKindCd">

<xPath>concat(rollNbr, '|', nbr)</xPath>
 <selectionRule>NODES_FROM_SHEET</selectionRule>





I am not clear what above code means and what it does. please advise.

Any good tutorials, resources, practice links, quiz links, video tutorials on these topics. Thanks in advance