Suppose you have a terminal with two tabs. You click, drag, and let go one of these tabs and it becomes an independent terminal window. After a while, you want to take this terminal window and reattach it as a tab to the previous window. How do you do it ? I have seen the merge windows option in the menus, but that merges all windows into one, which I would then have to revert.
You need to, in the window you want to move, go to View-> Show Tab Bar (if the tab bar isn't showing already).
Then, drag the tab of the window you want to move onto the window you want to move it to.