This is the 4852nd most frequent Russian word.
интенсивные
"Интенсивные" means "intensive."
Here, "интенсивные" describes the workouts, emphasizing their demanding and energy-consuming nature.
Интенсивные тренировки требуют много усилий.
Intensive workouts require a lot of effort.
In this sentence, "интенсивные" modifies "курсы" to indicate that the courses were focused and time-compressed.
У нас были интенсивные курсы перед экзаменом.
We had intensive courses before the exam.
Here, "интенсивного" qualifies "дождя," referring to the heavy and continuous nature of the rain.