Terminal lets you run commands in your project instance. Anything that can be run in a Ubuntu docker container can be run here.
You can open new terminal windows by clicking on the plus icons. All terminals will be shown as tab buttons on top of the terminal. Clicking on a tab will switch to that terminal window. You can close a terminal by clicking on the close button in the terminal tab button. At least one terminal tab will be open all the time. So the close button will not show if there is only one tab.