Tabs allow you to visually organize your fields. That means that tabs do not change data structure in any way, it only groups your fields together so that when viewing records through the dashboard, you can view these tabs separately.
You can create/select a tab inside the field configuration dialog window under the advanced option.
Once you have created a tab and saved a field, your tab is created and can be viewed inside the Tabs tab on your collection page. There you can reorder, move, and delete your tabs.
Your tabs can be seen in the record page. Here fields under the Address tab are shown together. Other fields will be in other tabs or a default tab Other if no tab for a field is selected.