It is a loan secured by a charge on the property. If they already have a mortgage or secured loan on the property that will be the primary or first charge. Any other loan secured on the property becomes a second charge, third charge and so on. Before a property can have a completed sale the registered charges must all be satisfied. Any money that is left over after these charges and settlement costs have been satified belongs to the (ex) homeowner.
If your parents have sufficient equity in their home or availibility of other funds to satisfy this or these loans, as well as providing for the deposit or purchase of their next home, then they can move as they wish. Sometimes people can consolidate a number of previous loans, into one new one on either their existing property or on the new home. However it is important to understand that the secured loan will represent a legal registered charge on the property, and it cannot be disposed of without the lenders consent or a court order.
It is also a point to understand that although the lenders security will have a hierarchy depending on wether it is a first or second charge etc. any of them can force a sale to realize their security if you or your parents do not keep up the repayment schedule. As this could mean the loss of their home, it is not something to be taken lightly, and I am sure your parents won't.