Custom Label in Lightning Component:
$Label.c.labelName for the default namespace.
$Label.namespace.labelName if your org has a namespace, or to access a label in a managed package.

<aura:component implements="flexipage:availableForAllPageTypes">
    <div onclick="{!c.clickLabel}">
        <ui:outputText value="{!$Label.c.Message}" />
    </div>
</aura:component>

Custom Label in Javascript:
$A.get(“$Label.c.labelName”) for the default namespace.
$A.get(“$Label.namespace.labelName”) if your org has a namespace, or to access a label in a managed package.

({
    clickLabel : function(component, event, helper) {
        var label = $A.get("$Label.c.Message");
        alert(label);
    }
})