The adjective internal describes something on the inside. If you're tempted by a second piece of cake, you may have an internal battle with yourself over whether you should eat the cake or not.
Internal can also describe something that takes place within a country or a group. You pay your taxes to the Internal Revenue Service, and politicians argue about how to spend your tax dollars when they discuss the country's internal affairs. You may have to write internal reports for your boss––reports that aren't supposed to be seen outside the company.