Libgdx makes use of several other libraries to interface the specifics of each platform in order to provide cross-platform support for your applications. Generally, a backend is what enables Libgdx to access the corresponding platform functionalities when one of the abstracted (platform-independent) Libgdx methods is called. For example, drawing an image to the upper-left corner of the screen, playing a sound file at a volume of 80 percent, or reading and writing from/to a file.
Libgdx currently provides the following three backends:
LWJGL (Lightweight Java Game Library)
As already mentioned in Chapter 1, Introduction to Libgdx and Project Setup, there will also be an iOS backend in the near future.