Introverted intuition is an introverted and a perceiving function. Ni, unlike extraverted intuition (which works more divergently), works convergently, meaning that Ni users are good at narrowing down existing possibilities to make one final decision. Ni users use their Se (extraverted sensing) in order to gather information from their external environment. They then use this information in order to create an interpretation on what is happening and what might happen next. Ni users typically make fewer, but more long-term, connections than Ne users.
While for Ne (extraverted intuition) users, coming up with ideas is quite an active process, Ni users tend to have insights that seem to come from nowhere (in fact, their processing is just more subconsious or passive). I would describe this as those 'aha' moments, when information suddenly just clicks into place. Another thing I've noticed with me (INTJ) and an ENTP friend of mine is that while they are good at brainstorming and coming up with ideas on the spot, I'm not as good, since my ideas tend to come to me at random moments, after my brain has done some processing.
Author's note: Ni is often portrayed as some magical psychic ability, with Ni users being able to just magically see into the future. And while we can be quite good at predicting what's to come, this is based on details in our external environment, and while it can feel like our insights come from nowhere, they are based on reality, or at least what our senses told us about reality (I believe our senses are fundamentally faulty but let's not go into that). For me, Ni just means that I often predict plot twists in media such as TV and books, and feel less excited when the plot twist is actually revealed, because I saw it coming all along. And of course, I can never tell whether I'm using my Ni or just being an overthinker.