Tabs are a special toolbar that provides quick access to the editor windows in your layout from the scene view. Tabs are meant to let you work contextually right from the scene with reduced visual obstruction.
By default, tabs are only shown when the scene view is maximized (full screen), but that can be changed from the Peek preferences to be either always or never displayed.
Popup tabs behave like the tabs in a web browser: only a single one of them can be open at once. They always appear next to the tabs toolbar and cannot be moved or resized. By default, they will take the size they had in your layout.
Pinned tabs behave like a floating workspace: you can open multiple of them at once and move or resize them around the screen. If you resize a pinned tab, that size will be remembered next time you open it in a popup.