This is where you choose a subarea or a sub-timeseries instead of plotting the whole file. A subarea is defined by the grid numbers on the spatial axes and a sub-timeseries by the time step numbers.
Instead of selecting a subarea now, you can also choose to zoom-in in the Popup Menu. Choosing it now will probably cause less memory problems for your computer, and this will be the area that is saved to the output file.