swift difference between final var and non-final var | final let and non-final let

The final modifier is described in the Swift Language Reference, which says

final

Apply this modifier to a class or to a property, method, or subscript member of a class. It’s applied to a class to indicate that the class can’t be subclassed. It’s applied to a property, method, or subscript of a class to indicate that a class member can’t be overridden in any subclass.

This means without final we can write:

class A {
    var x: Int {return 5}
}
class B : A {
    override var x: Int {return 3}
}
var b = B()
assert(b.x == 3)

but if we use final in class A

class A {
    final var x: Int {return 5}
}
class B : A {
    // COMPILER ERROR
    override var x: Int {return 3}
}

then this happens:

$ swift final.swift 
final.swift:6:18: error: var overrides a 'final' var
    override var x: Int {return 3}
             ^
final.swift:2:15: note: overridden declaration is here
    final var x: Int {return 5}

Final variables can't be overridden in subclasses. It also hints this to the compiler which allows it to inline the variable. It other words every time the compiler sees a final variable being used somewhere, it can immediately substitute the value. Whether or not the compiler actually does this is up to the compiler and whatever optimisations it knows/uses.