To irrigate is to supply with water, usually with ditches and channels that allow the water to flow.
Irrigating is a type of watering that is important in one field in particular: farming. If a farmer has a huge number of crops, you can see how getting water to them could be difficult. The farmer irrigates by digging little channels that allow hard-to-reach spots to receive water. If there is a drought, irrigating will have to happen often to save the crops. People with large gardens might need to irrigate as well. Irrigating waters plants, keeping them alive.