The program comes with a number of built-in items of all sorts including ground textures, vegetation, buildings, industries, track, signals and rolling stock. There are also some built-in layouts and activities.
The program includes facilities for creating your own layouts (also called routes). You can adjust the height of the ground, add ground textures and all the other features. This is done in an option called Surveyor.
A large range of additional items can be downloaded from the publisher's server, or from a range of fansites. Some is free, some is not, some is downloaded faster if you pay.
With enough skill it's possible to make additional items yourself using a 3D modelling program such as gmax. Most people find this quite a difficult challenge to learn though.
It's also possible to create activities of different kinds.
There are various versions of Trainz available and we tend to have very strong and varied opinions as to which is best. Not wishing to reopen that debate here I'll just say take a look at some of the threads on this forum and check very carefully at the product descriptions to make sure you buy a version which will do what you want.
Have fun!
John