Most likely, im = "input message" (not "instant message").
This aligns with OpenAI's ChatML (Chat Markup Language) conventions, where:
im_start = Start of a user or assistant input.
im_end = End of that input.
These tokens help the model parse structured conversations, especially in multi-turn chats.

Why Not "Instant Message"?
Instant message" (IM) is an older term (like in AIM or MSN Messenger), but in this context, it doesn't fit the structured role of these tokens.The tokens are more about demarcating inputs rather than describing real-time messaging.