This is how a wetland is made:
Imagine the ice cream container is the wetland and the newspaper is rocks and stones. If the ice cream container is empty and has holes, all the water will drain out. If it has newspaper (rocks and plants) in and around it, they will soak up the water and it will escape more slowly.
When it rains, the water goes to the wetlands and is kept there by the rocks and plants. It goes there because the wetland ground is lower than all the ground around it. It slowly drains out and waters the ground around it.