app/code/Foo/Bar/etc/config.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
    <!-- This is the "config type" and is "default", "stores" or "websites" -->
    <default>
        <!-- Every config reference is just a key:value store -->
        <key>value</key>
        <!-- An optional path can and is usually set by wrapping nodes -->
        <some>
            <arbitrary>
                <!-- This config key will have the path some/arbitrary/key -->
                <key>value</key>
            </arbitrary>
        </some>
    </default>
    <!--
    If the config type is "stores" or "websites", the corresponding child node
    is used to determine the scope of the configuration value
    -->
    <websites>
        <!-- The config here will only apply to the "base" website -->
        <base>
            <key>value</key>
        </base>
    </websites>
</config>