PLEASE NOTE: SPOILERS AHEAD
Communion is based on actual events that author Whitley Strieber was an unwilling participant in. At least, he was unwilling at the start.
First, a little bit about writer Whitley Strieber. Strieber was a novelist far before writing Communion. He was most known for writing horror novels, such as The Wolfen and The Hunger, both of which you may also know from their film adaptations. As I stated, Strieber was known for writing horror fiction. Communion, on the other hand, is a work of non-fiction.
The film begins in the fall of 1985; writer Whitley Strieber was having a bit of what some would refer to as "writer's block." He decided to take his family from their home in New York City, and spend some time at their vacation cabin in upstate New York. He was hoping time away from the hustle and bustle of the big city might help him to come up with some new thoughts or ideas for his next book. While on this getaway vacation, he was awakened to see the face of what could only be described as an "other worldly" intelligence. He was being visited by aliens from another world. When he woke the next morning, he had no memory of this event. He and his family would return to New York City, where he later began to have terrifying hallucinations and decided to take his wife's advice and seek out the help of a psychiatrist.
The psychiatrist convinced Whitley to undergo hypnotic regression therapy, where they discovered that he had been visited and abducted by these alien life forms for most of his life. Whitley is unable to come to grips with all of this, and so he returns to his upstate cabin and ultimately is abducted yet again by these creatures — and this time he is able to communicate with them. Although he does not truly learn who or what they are, nor where they are from, he does learn that it seems these beings are communicating with humans to help them transform and perhaps better themselves. They may be sort of "pushing" or guiding people in the right direction in their lives.