A nuclear reactor is the part of a nuclear plant that generates steam from the radioactive materials.

NuScale's plan, for example, is to build a pre-fabricated reactor you can ship via truck to the plant. You put it in a deep pool, and add some piping to connect it to your steam turbine, and you've got a power plant.

It's modular, in that you can put many nuclear reactors in your pool. You can hook them up to whatever steam turbine you want. You don't necessarily need more sites, you can have one site with more reactors.

The advantages of the design is better passive emergency safety, centralized building of the most complex parts, and the ability to build smaller plants for smaller cities.

Additionally, there's been some discussion refurbishing old coal plants with small modular reactors; you'd basically replace the old coal furnace with a new pool of SMRs, hooking the steam to the old turbines and other infrastructure. Honestly, I'll beleive it when I see it.