Those are Tab Groups instead of Tabs. It's a new way to organize tabs across Safari and your devices in recent OS updates.
You don't have to use them though! It might be that you're opening tabs in Tab Groups. For example, if you Control-click a link and select Open Link in Tab Group, and then New Group, it will create a new Tab Group with that tab. You can instead use Open Link in New Tab or Open Link in New Window, as needed. The other way you might be creating tab groups is by clicking the New Tab Group button instead of the New Tab button:
New Tab Group button
New Tab button (typically found at the top right of the Safari window)
For a more in-depth guide on Tab Groups, see Group tabs in Safari on Mac - Apple Support.