Html (as principais classes são tooltip-toggle e tooltip-content):
<span class="my-tooltip">
<a href="#" class="tooltip-toggle">Hover?</a>
<span class="tooltip-content">Details here</span>
</span>
Menos:
.my-tooltip {
.lib-tooltip(right);
}
Detalhes adicionais:
https://github.com/magento/magento2/blob/develop/lib/web/css/docs/tooltips.html