A number of methods of preserving food existed in medieval times:
Keeping food alive
In 12th Century homes, It was considered desirable to have a 'vivarium' in which live fish might be kept until it was time to eat them. (Holmes 1952 p93)
Many foods can be dried (like Beef Jerky).
Many foods can be salted and preserved, this works well with meats and fish.
In brine solution
Fish might be pickled in brine (salt water) before use. (holmes 1952 p93)
Many Fish can be smoked, such as salmon and many kinds of meat.