It's numeric index of the current entity (the entity your script is assigned to).
g_Entity is global array of entities. g_Entity[e] returns current entity, or in other words, another array of entity properties. For example, g_Entity[e]['x'] will return current entity's X coordinate.
Your scripts are always defined as "function script_main(e)", or whatever_main(e). "e" is provided by FPSC:R engine when it calls your "script_main" function.
