ArmA2.RPT file is the debug report file created by the game engine when its running. Sometimes if you have a lot of errors in addons, terrains or missions this file gets excessively large (several megabytes in fact). However it is a valuable tool for editing as it reports errors in so much detail.
Your RPT file is located in the root directory where the Users/ directory is located. How to configure users directory, please refer to -profiles parameter.
Please refer to arma.RPT page for some of the possible errors messages seen in RPT file.