A multimap allows multiple values for the same key. Insert the same key with different values and all entries stay. Unlike regular maps, keys can have multiple associated values. You can't use mm[key] because a key might have multiple values.
Instead, use equal_range(key) to get a pair of iterators covering all entries with that key in the multimap. Multimaps are less commonly used than regular maps. Often a map<Key, vector> is clearer when you need one-to-many relationships between keys and values.