'Intensive courses' are those that try to make maximum progress in minimum time; they usually involve several hours of learning every day rather than the standard couple of hours once or twice a week.
Yes, this is what it is. I took an intensive Spanish course in Mexico in 2000. I was in a learning situation with a teacher and other students 5 hours a day, 5 days a week for 4 weeks. And then I spent the rest of the day learning the language by talking to my host family, buying things at the store, etc. I learned a whole lot more in that 4-week intensive course than I would have back home in a 12-week extensive course with only 3-6 hours of language teaching/learning and practice per week.