Hi Felipe!
DIP is the acronym for device-independent pixel. It is a abstraction of the pixel dimensions, so that designs maintain proportions across devices of different screen densities (dpi values).
So for instance a 1 dip border will render as a 1px on an SD(1024x768) iPad, and as 2px on a Retina iPad, thus maintaining a similar physical size/ look to the end-user. Given the wildly different screen densities of phones / tablets, this unit aims to maintain a more consistent look without having to specify exact pixel dimensions for each of the devices.
