which program are you using to encode?
the exe path is the location of the program. Something along the lines of C:\program files\program xyz\glorious_program.exe
The aspect ratio is width/height. (640/480 = 4:3 = 4/3 = 1.333333)
You may want to Read ErMaC & AbsoluteDestiny's Friendly AMV Guides Lovingly Overhauled Largely by Zarxrax
it's got sections on avisynth, aspect ratios and more. It's not the end-all guide for the technical aspects of videos, but it's a good start.