A bag or multiset is a collection of object whose members need not be distinct (unlike a set, whose members are distinct).
So the bag [ a, b, c, c ] is the same as the bag [ c, a, b, c ] but distinct from the bag [ a, b, c ]. Contrast this with the set { a, b, c, c }, which is the same as the set { c, a, b, c }, and is also the same as the set { a, b, c }.
The number of repetitions of each member is the multiplicity of that member. For the bag [ a, b, c, c ], the multiplicity of a and of b is 1, and the multiplicity of c is 2.