Intense is used to describe something that is very great or extreme in strength or degree.

If you describe an activity as intense, you mean that it is very serious and concentrated, and often involves doing a great deal in a short time.

If you describe the way someone looks at you as intense, you mean that they look at you very directly and seem to know what you are thinking or feeling.

If you describe a person as intense, you mean that they appear to concentrate very hard on everything that they do, and they feel and show their emotions in a very extreme way.