It means that model was tuned to to act as chat bot. So write a reply on behalf of assistant and stop generating (by inserting special "end of turn" token to signal inference engine to stop generation).

Base model (without instruction/chat tuning) just generates text non stop ("autocomplete on steroids") and text is not necessarily even formatted as chat -- most text in training data isn't dialogue, after all.

Use the it versions. The other versions are base models without post-training. E.g. base models are trained to regurgitate raw wikipedia, books, etc. Then these base models are post-trained into instruction-tuned models where they learn to act as a chat assistant.