This is the dialog you see when you open GIMP for the first time.
There is probably no problem to just agree to all of the defaults. However, the GIMP installation can use some tweaking here and there to make it appear and behave perfectly for you. Some of these options are set during this dialog.
Since this window mentions the GNU General Public License you know it is truly a Welcome dialog you are entering into. ;) (screenshot)
Also, note the "Continue" button. GIMP does not even ask that you agree to it, merely to continue. Feel free to press the continue button. (screenshot)
I have always left this as is (screenshot).
This window shows you the files that GIMP will make. It will have some complaints if you told it to install some place that it didn't have permission to be. There is a scroll bar to see all the things GIMP has for you. (screenshot).
Setting your memory usage is not an easy thing. So much depends on what your needs are for GIMP and what hardware you have to work with. You have two options at this point. Go with the default value the developers have set here, or determine the best value. A brief tile-cache explanation. might help you determine this value. Most users will want to set this value to about 80% of the memory available on their systems. The tile-cache information might also be helpful to you if you are encountering memory problems when using the gimp. (screenshot)
/tmp might be a good place for the swap.
Monitor Resolution is the ratio of pixels, horizontally and vertically (screenshot).
My monitor was impressively off when I tried the Calibrate Dialog The Calibrate Game is fun to play. You will need a soft ruler. (screenshot).