Drainage
Drainage is how water flows away through gaps, pipes, or soil.
Think of it this way
Water drains from a colander the same way it drains through soil. You fill the colander with pasta and water. The water flows down through the small holes and into the sink below. Solid pieces stay behind, but liquid passes through the gaps.
Explaining drainage by grade level
Rain falls on the ground and needs a place to go. It flows through holes and gaps to move away. A dome drain cover has space under it for water to pass. A flat cover gets blocked by leaves much faster.