So, imagine you have a little pool of water in your backyard. And let's say you have a hose that you use to add more water to the pool. The more water you add, the bigger the pool gets, right?
Now, imagine that your backyard pool is actually the mouth of a river that flows into the ocean. And instead of a hose, you have the tides of the ocean coming in and out of the mouth of the river.
The tidal prism (don't worry about the big name, it's just a fancy way of saying "the amount of water that comes in and out with the tides") is the amount of water that moves in and out of the river mouth with each tide.
If the tidal prism is big, that means a lot of water is coming in and out with the tides, and the river mouth is open wide. This can help keep the river healthy and provide a good habitat for fish and other creatures.
But if the tidal prism is small, that means not as much water is coming in and out with the tides, and the river mouth is narrower. This can cause problems, like making the water too salty or creating a buildup of sediment that can hurt the plants and animals that live in the river.
So basically, the tidal prism is just a way to measure how much water is moving in and out of a river mouth with the tides. The bigger the tidal prism, the happier the river (and the fish and plants that live there) will be!