Assumptions are beliefs taken for granted. They are so natural and involuntary they usually do not enter our consciousness. We assume that when we get out of bed, the floor will be beneath us and that when we mail a letter the intended recipient will receive it.
However, there are also assumptions we make that may not be valid. For example, we may assume that someone is angry with us by the manner in which that person speaks to us. Yet it could be that the person is feeling frustrated by an event entirely unconnected to us and is simply “taking it out” on the first person encountered—in this case, you.