Ok, so imagine you were expected to strip naked, cuddle up to someone else who was also stripped naked. Imagine you are expected to pretend to have very real and intimate feelings with one another and in the process do very intimate things with them like kiss, touch their body, and simulate penetrative sex all while being in a room full of people watching you do this and with cameras recording the entire thing with the intent to show it to the world. That in itself would make most people at least a bit uncomfortable.
Now, maybe you also feel uncomfortable because you have a spouse or significant other. Maybe you know this other person well and they are platonic friends. Maybe they are barely an acquaintance, just a work colleague. Maybe you have romantic feelings for this other person. Maybe you hate them. Maybe the other person has made advances on you. Maybe you have a history of being assaulted or of being used sexually. Maybe you are self conscious about your body. For any number of very justifiable reasons, this situation can be anywhere from slightly to very uncomfortable for either or both actors, even scary in some situations.
The intimacy coordinator is not a sex coach. They are there to make sure the situation is safe, consensual, and as respectful and private as possible for both actors, and to see that their needs are met for this very stressful circumstance to minimize discomfort. They also make sure that nobody on set oversteps boundaries, that the actors' rights are upheld, act as the single point of contact between the actors and production to minimize uncomfortable conversations and miscommunication. They will be involved in choreography of the scene, but not as a coach so to speak, but rather in mentally preparing and making exactly clear the understanding for all parties what is going to happen, giving everyone the time to process and veto plans if necessary.