When you are adding a carousel, you have an option to add a Button along with it. When you add a button, you can configure it to various types. It can be an Open screen, text, call, etc type of a button.
In this case, if you have added an Open Screen Button Type, and have added Screen to open as Link, then you would see this toggle Launch in app toggle, as shown.
What this toggle does is, if you enable it, then any link added in the Link to open field will be loaded in the bot itself, as shown.
If you disable this toggle, then the link will open in another tab.