Highly means to a great degree or extent. When something is described as highly unlikely, it's very unlikely to happen. When someone is highly skilled, they possess an impressive level of ability. When a teacher is highly respected, many people admire and trust her.

The word adds emphasis and shows intensity. A student might be highly motivated to finish a project, meaning their drive goes beyond ordinary interest. A scientist could be highly regarded in her field, indicating widespread recognition of her expertise. A book might be highly recommended, suggesting many people think it's worth reading.

Notice that highly typically pairs with positive qualities or desirable outcomes: highly intelligent, highly successful, highly effective. You wouldn't usually say someone is highly average because highly suggests standing out, not blending in.