Economics of new nuclear power plants is about how much money it would cost to build and run a new nuclear power plant, and how much money it would make by selling the power it produces.
Building a nuclear power plant is expensive - it can cost billions of dollars - because the materials and equipment required are very expensive. To run the plant, it takes a lot of energy to run the nuclear reactions to produce power. This energy costs money too.
When the power plant is running, it will produce a lot of power. This power can be sold to people who need it. The money made from selling this power can offset the costs of building and running the power plant, so that in the end, it can make money.