A RenderTexture is just a Texture that you can also draw to. One way to use it is to let you replace the contents of a Texture with another. You create your RenderTexture, e.g.
rt = new RenderTexture(1024, 1024);
You can then draw onto it, like you render to screen. E.g.
When you want it to use another texture you just draw that to it:
rt is a RenderTexture, but that also means it is a Texture and can be used anywhere you use a Texture.
You can do a lot more than this. In particular you can treat the RenderTexure as a TextureAtlas, only updating parts that have changed not all of it.