Barrier is a magic spell in Final Fantasy VII Rebirth. It is the basic spell provided by 
Barrier Materia. It grants the 
Barrier effect, which halves physical damage taken for its duration.
Though 
Protect also exists, it has a different function, wherein Protect simply improves the user's 
Defense while Barrier halves the incoming damage. The two effects are multiplicative of each other.
Barrier is also used by Moss-Grown Adamantoise.
Use[]
Barrier costs 11 MP to cast. It grants a protective status that halves physical damage for its duration, and is multiplicative of other defenses against physical damage. The spell is fairly expensive, but can be vital against physical attacks that may otherwise kill party members.
Barrier is outclassed by Manawall, which also applies the 
Manaward. Manawall, however, is much more expensive to cast, meaning players may prefer to simply cast Barrier if they only need to protect against physical damage. Protect also has a similar role to Barrier, but is cheaper to cast; in some cases, players may wish to cast both. 
Shield eclipses all other physical damage-reduction means, but is harder to deliberately use when needed, as it is an effect from a limit break and Lucky Slots.
Barrier can be used effectively when barrier materia is paired with 
Magnify Materia. This allows the user to apply the status to all party members at once. In many cases this is a perfect use case for magnify materia.
Barrier can be used by Barret to great effects. His Lifesaver ability diverts damage intended for other party members onto himself, but reducing their damage taken means he will also take reduced damage from them.