What does we must take care of the earths water so the earth's water can take care of us mean?

It could mean either of two very important things, depending on whether it refers to "fresh water" or to "all water."

We shouldn't waste or pollute the very small portion of Earth's water that is fresh water: we use that water to drink, and to grow and process food. If we pollute our water, or use it unwisely, there will not be enough water for all of the things we need it for.

We also should not pollute or damage the Earth's oceans, because human existence is based on a complete ecosystem, including oceans, as well as the water cycle. Marine plants still produce the majority of Earth's oxygen.