What does "unconscious memory" mean?
Although we usually refer to memory to describe our ability to consciously remember past experiences, in neuroscience this term is also used to incorporate unconscious influences of our past on our current perceptions and actions.
In other words, sometimes/in some instances, we are not aware of the influences of our past on our way to remember things.


  These unconscious influences on our current behaviour depend on what
  has been called implicit memory or nondeclarative memory that
  can exist in individuals with damage to the hippocampal memory
  system (centre of declarative or explicit / conscious memory).


This example is often made: if you try to describe how you learned to drive a stick-shift car,it is likely that you wouldn't do a good job of consciously reconstructing the chain of sequences that allowed you to learn how to drive a stick-shift car... Nevertheless, your behaviour would be a clear indication of you training.

Memory is not a single entity but consists of multiple processes or systems



References:
Emotional memory across the lifespan Elizabeth A. Kensinger