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.

good old illustrtation: https://www.ml6.eu/en/blog/large-language-models-to-fine-tun...

The it- one is the yellow smiling dot, the pt- is the rightmost monster head.