The files to restart a trajectory in cp2k are just input files with all the defaults made explicit. So it should not be so difficult to understand them.
Often one wants to continue a MD trajectory with slightly different parameters, for example, an equilibration run followed by a NVE run. This is done with the use of a restart file, however it can be difficult to edit or use this file directly. To make this easier one can prepare an input that takes part of its values from another input. In particular it might take positions and velocities from the restart file.
This is done with the section EXT_RESTART giving the name of the external input with the EXTERNAL_FILE_NAME keyword. In this section there are keywords that controls exactly what is taken from the external file, for example if one wants to restart just the positions and not the velocities he should set RESTART_VEL FALSE.
In DFT calculation there is also another restart, the one of the wave-functions. This restart is stored in a binary file that is architecture specific (see cp2k/tools/RestartTools if you need to transfer it). The name and location of this file are controlled by FORCE_EVAL%DFT%WFN_RESTART_FILE_NAME.