Can you please explain what this mod does?

To quote the text below the info box:

Shows Mini Trains in some background simulations.

In other words, it (currently) adds five menu simulations to the game where https://mods.factorio.com/mod/Mini_Trains are used.

Do I have to to additional stuff, like disabling original ones, etc?

The mod only adds simulations. If you want to disable the original ones there are additional mods which achieve that.

The 1.1 version contains entirely new simulations. For the 2.0 version the mini trains collisions were changed so these need to be recreated and I currently lack the time to do so and only included vanilla simulations where I switched the trains.