In short
An annotation is a note or restriction entered on a property's land registry record that binds third parties. A mortgage is a right established over the property as security for a debt and registered on the deed.
The declarations, annotations and encumbrances sections of the registry record carry the property's unseen history. Entries such as mortgages, attachments, promises to sell, lease annotations, usufruct rights or family residence annotations may appear here. Buying a property can mean taking on these entries with it.
A mortgaged property can be sold; there is no legal obstacle. But the mortgage does not lift by itself on sale. In practice there are two routes: part of the sale price clears the debt and a release is obtained, or the buyer knowingly takes the property on with the mortgage. Which route applies must be written into the contract.
Why it matters to an investor
No purchase made without seeing the registry record is safe. However good a property's photographs, location and price, a single attachment annotation on its record can halt the transaction entirely. Checking the record should be the first step of a purchase, not the last.