Highly means to a high degree, extremely. A highly dangerous substance is one to stay far away from. If you think very highly of yourself, you give yourself the thumbs up every time you walk by a mirror.
Highly comes from the Old English word healice for basically, "honorably," but has also come to mean “very very.” A highly respected writer is one people look up to. A highly regarded politician will probably win an election. Highly also refers to anything that is intense or extreme. A highly flammable chemical can burst into flames easily. The -ly ending is highly common for an adverb.