Something that is unknown or unfamiliar can have many meanings, such as:
Something or someone in your waking life you don't know or perhaps would like to know or understand better
A vague feeling, thought, or idea within yourself
A general threat or fear that's on your mind
Something that is not central to the dream's meaning, but possibly just plays a role in setting context or recreating a past event or setting
For clues to meaning, consider any characteristics of the unknown thing, what stands out about it, and your emotional reaction to it.