In One Sentence
AEO (Answer Engine Optimization) refers to tactics for getting your information displayed in the "answer" position that appears directly at the top of search results.
What does this look like in practice?
For example, if you search Google for "What is the distance from Tokyo to Osaka?", the very top of the search results shows the answer directly: "About 515 km".
The goal of AEO is to get your company's information displayed in that "direct answer position".
Another example: a health-food company's site getting cited as "100 mg (recommended for adults)" for a query like "What is the recommended daily intake of Vitamin C?".