Okay kiddo, a dyke is like a big wall that people make to keep water away. Imagine you have a sandcastle on the beach and the water keeps coming up and knocking it down. So you build a big wall with sand to keep the water from getting to your sandcastle.
A dyke is like that, but it's a big wall made of dirt and rocks that people build in places where the water might flood, like near a river or the ocean. The dyke stops the water from going where we don't want it to go.
Sometimes people also use the word "dyke" to talk about a woman who likes other women, but that's not a very nice way to use the word. We should always be kind to people no matter who they like.