Cura is a "slicing" software that sets the options for a 3d printer to follow and then creates detailed instructions for the printer by creating a "g-code" file. This file can be saved to a small memory and transferred to the printer.
Cura allows you to arrange, orient, and re-size your design file before printing. It also has option to automatically calculate things such as the top- and bottom-layer thickness and how much in-fill your printed object needs.
Note that Cura is not software that can be used to design a model, it is basically a program that takes a final design and translates it into the printer's language.
For more information on Cura, please see the online instruction manual.
Cura is open-source software, you can download and install it for free at https://ultimaker.com/en/products/software.