The term incumber refers to the burdening of real estate with legal claims or debts. Mortgages are a prime example when an interest in property may be incumbered. Liens on property form another example.
Incumbered assets are held by one party, but are usually open to claim by another. It is also required by the party owning the incumbered asset to first satisfy all debts or liens before selling the asset. For example, a homeowner can't legally sell his or her house until he pays off the mortgage (unless the buying price would satisfy the outstanding mortgage).