I think the best way to explain something that few people experience is by comparing it to something others are more likely to have experienced so that they are more empathetic to your situation and how you may or may not be coping.
To explain auditory hallucinations (for me, this means voices) I have used the comparison of listening to music through headphones. The noise is coming through your ears and feels as though it's filling up your head. You can hear it but no one else can and sometimes it's so loud you worry others will hear. I experienced hearing voices for three years (six, now in total) before the visual hallucinations began, so I had a long time of professionals being unable to understand what I was going through. The main problem with this, is that when you self-harm or attempt suicide because of this thing they don't understand and have no empathy for, it's almost understandable to be met with ignorance and a bad attitude. No excuse but almost understandable.
Visual hallucinations, are something I've experienced for the past three years and I've personally found them to be something more widely comprehended because how many people do you hear saying 'oh I thought I saw something out the corner of my eye'? For many people, this is what they liken a visual hallucination to. Not those who experience such hallucinations though. Mostly those who have someone tell them they're hallucinating and they say 'oh this time I thought I saw something but looked again and there was nothing there, is it like that?' No, it is not. Imagine someone asking about your childhood and you tell them how you had a family dog and have lots of fun memories with it. Your family tell you there was never a dog. You remember playing in a paddling pool with it, taking it for muddy walks in fields... But it was never there. It's strange and at first you're confused and are desperate for proof that it hadn't been there. And then the acceptance sets in. And with the acceptance that something you thought was there, wasn't really, you begin to question all of your childhood memories. Or even the dog that you currently own, does he exist? Can everyone see him?! This, is how I'd help someone to understand a visual hallucination.
I've only experienced other hallucinations a handful of times so I can't advise on these but I hope this post might have inspired other ideas of how to help those who care understand what you're going through.