Something is versus the outside. Back in the day, everything was a battle with the outside. You had the Mongols... the rebels... and the foreigners. So, when you say something is "versus the outside" you're just saying it's something compared to where you are right now. It's something that's foreign or external.
This is a formal word typically used at the beginning of compounds to indicate that the subject is related to foreign affairs, such as 対外政策 (foreign policy) or 対外貿易 (foreign trade).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
This company is focusing on business overseas.
The president always says he wants to improve our public image.
There is no concern about external communication.