Okay kiddo, so a fjord lake is like a really big lake that's extra special because it's formed in a special way. You know how we sometimes see big tall mountains with steep sides that look like they go straight up into the sky? Well, fjord lakes are formed in places where those mountains meet the sea.
You see, a long long time ago, there were huge glaciers (that's like a really really really big pile of ice) that covered the land. These glaciers would slowly move and scrape the land, making really big grooves in the mountains. When the climate began to warm up, the ice began to melt and that water goes down the grooves in the mountains, creating rivers. These rivers of melted ice would carry small rocks and dirt back down the mountain and towards the sea. Over time, these rivers carved out big valleys in the mountains, and as the sea levels rose, the valleys began to fill up with water- thus creating fjord lakes.
So, imagine you have a big mountain that goes straight up into the sky, and it has this valley that is really really deep. This valley is then filled with water, creating a lake. And because of the way it was formed from the glaciers and moving ice- it's called a special name- a fjord lake.
Isn't that interesting? Even big parts of nature can come together in ways that make really amazing sights for us to see.