Device tree is a set of scripts, kernel and files (called proprietary blobs). Those scripts controls the way of building Android for a specific device. For example, to put proper graphics drivers in to proper folder in Android tree after building. Scripts also responsible for telling build programs(compilers and linkers) to for example don't include in our build NFC related stuff because we don't need it. Or to include custom program that will be linked with Settings, and for example will switch internal/external storage. There is also a kernel tree with is also building all together with rest of Android. Kernel is hardware related program and it controls all drivers, memory, lcd's, wifi, bluetooth etc. stuff. It is a core system.
Go to OP and find link to Surdu's GitHub, where You can see His device tree for P6