What happens when you see visions in the future?

In most cases, nothing at all happens when you "see visions of the future." If by "visions," the question refers to images seen in dreams while sleeping, then those visions merely illustrate your own worries (if frightening) or wishes (if joyful.) Either way, there is nothing supernatural involved.

If the question refers to "visions" seen while wide awake - either seeing or hearing something that no one else can see or hear, then these are not visions but delusions. Delusions indicate that you have a medical problem that requires prompt attention by a medical professional. The cause may be as simple as a migraine headache or as serious as a brain tumor, and needs immediate attention.