Freezing will prevent bacteria from multiplying but it does not destroy them. Once frozen food has been thawed, the bacteria may multiply with renewed energy. If food is frozen and thawed repeatedly, bacteria could reach dangerous levels.
You can refreeze food safely if it has been cooked after thawing. However, freezing also results in some loss of moisture and if food is repeatedly frozen and thawed, it will become unpleasantly dry. Fibrous food like meat also loses its texture as it thaws, and will become flabby if it is refrozen and thawed again, say in a casserole.