Internally, all objects members are stored in slots. They are contiguous starting from 0. SE_INDEX lets you access a member by its slot.
The usual use for this method of accessing members is to iterate over all members of an object. The ScriptEase API call seObjectMemberCount will tell you how many members an object has, and thus how many slots it is using. Those slots are numbered from 0 to one less than the number of slots.
Note that the slot a particular member uses will changes as members are added to or removed from an object. Do not try to use SE.INDEX to access regular members or assume they occupy any particular slot.