Rations were controlled portions of certain foodstuffs, distributed in times of emergency or scarcity, or in situations where carrying large amounts of food was impractical. Rations would often contain carefully balanced mixes of nutritional supplements designed to nourish the body in place of normal food. A common problem with such rations was that their taste left much to be desired.
Rationing in time of crisis or scarcity
Rationing food and water was a strategy employed in many cultures to deal with scarce supplies and shortages. If the situation grew worse, it might become necessary to cut the rations. Rationing was sometimes necessary on newly-colonized planets.
Field rations, combat rations and emergency rations
During away missions and many other types of deployments, rations were widely used to feed personnel, as it obviously not always possible to reach a mess or replicator for meals. While such rations were used for practical reasons rather than due to scarcity, often shuttles and isolated facilities would also contain emergency rations, which could keep a person alive until help arrived should they find themselves cut of from their support structure. This was standard practice in Starfleet and amongst many other cultures.
The Federation also made extensive use of rations, who went by many nicknames. Starfleet rations were specifically, "concentrated vitamin-enriched nutritional supplements." (VOY: "Innocence"). Although use of these rations were considered by many Starfleet officers to be a necessary evil, Julian Bashir developed a candy bar with superior nutritional value. (DS9: "The Siege")