This is not usually defined in the jseopt.h file, but rather by your compiler depending on if you are building a release or debug version. However, it is a very important define which ScriptEase uses extensively. When NDEBUG is not defined (i.e. a debug build), ScriptEase does a significant amount of internal checking for any bugs, either errors in ScriptEase itself or errors in an application's use of the API. You should do all development without the NDEBUG flag as you can find many bugs this way. When you are ready to release, turn NDEBUG back on for the fastest possible code.
JSE_MEM_DEBUG (on if NDEBUG not defined)