XML-to-JSON: what is a base element? and what is property?

I'm trying to figure out what the author means by "base element:"

"XML elements become properties of their base element. Suppose you had the following XML:"

<customer>
    <first>Bill</first>
    <last>Burke</last>
</customer>

The JSON mapping would look like:

{ "customer" :
    { "first" : { "$" : "Bill"},
       "last" : { "$" : "Burke" }
    }
}

I understand what's happening, but what does this person mean by base element? Actually, what does this whole sentence mean? "XML elements become properties of their base element." And what is a "property" is this json property or xml property? for xml there's elements, attributes, and values, and for json there's key and value. So what is this "property" thing?