Intensive agriculture seeks substantial production from repeatedly cultivated land through concentrated inputs, labour or management. Depending on the setting, this can involve irrigation, soil improvement, tools or other techniques. It is not simply another word for a physically large farm.
The distinction is useful when comparing how food is produced. A small, carefully managed plot can be intensive, while a large operation can use relatively few inputs per unit of land. Yields, resource use and environmental effects still need to be assessed separately.